Research Article |
Corresponding author: Mark A. Townley ( mark.townley@unh.edu ) Academic editor: Martin Husemann
© 2017 Mark A. Townley, Danilo Harms.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Townley MA, Harms D (2017) Comparative study of spinning field development in two species of araneophagic spiders (Araneae, Mimetidae, Australomimetus). Evolutionary Systematics 1: 47-75. https://doi.org/10.3897/evolsyst.1.14765
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External studies of spider spinning fields allow us to make inferences about internal silk gland biology, including what happens to silk glands when the spider molts. Such studies often focus on adults, but juveniles can provide additional insight on spinning apparatus development and character polarity. Here we document and describe spinning fields at all stadia in two species of pirate spider (Mimetidae: Australomimetus spinosus, A. djuka). Pirate spiders nest within the ecribellate orb-building spiders (Araneoidea), but are vagrant, araneophagic members that do not build prey-capture webs. Correspondingly, they lack aggregate and flagelliform silk glands (AG, FL), specialized for forming prey-capture lines in araneoid orb webs. However, occasional possible vestiges of an AG or FL spigot, as observed in one juvenile A. spinosus specimen, are consistent with secondary loss of AG and FL. By comparing spigots from one stadium to tartipores from the next stadium, silk glands can be divided into those that are tartipore-accommodated (T-A), and thus functional during proecdysis, and those that are not (non-T-A). Though evidence was more extensive in A. spinosus, it was likely true for both species that the number of non-T-A piriform silk glands (PI) was constant (two pairs) through all stadia, while numbers of T-API rose incrementally. The two species differed in that A. spinosus had T-A minor ampullate and aciniform silk glands (MiA, AC) that were absent in A. djuka. First instars of A. djuka, however, appeared to retain vestiges of T-AMiA spigots, consistent with a plesiomorphic state in which T-AMiA (called secondary MiA) are present. T-AAC have not previously been observed in Australomimetus and the arrangement of their spigots on posterior lateral spinnerets was unlike that seen thus far in other mimetid genera. Though new AC and T-API apparently form throughout much of a spider’s ontogeny, recurring spigot/tartipore arrangements indicated that AC and PI, after functioning during one stadium, were used again in each subsequent stadium (if non-T-A) or in alternate subsequent stadia (if T-A). In A. spinosus, sexual and geographic dimorphisms involving AC were noted. Cylindrical silk gland (CY) spigots were observed in mid-to-late juvenile, as well as adult, females of both species. Their use in juveniles, however, should not be assumed and only adult CY spigots had wide openings typical of mimetids. Neither species exhibited two pairs of modified PI spigots present in some adult male mimetids.
Pirate spider MimetidaeAustralomimetus spinneret spinning field molting silk gland tartipore
A common name applied to the family Mimetidae, pirate spiders, reflects their routine practice of invading the webs of other spiders and killing the occupant. Indeed, web-building spiders, including spider eggs (
Several recent cladistic analyses, based partly or solely on sequence data, agree in placing Mimetidae within Araneoidea, and further agree that mimetids are closely related to the orb-web-building family Tetragnathidae (
The family Mimetidae currently comprises some 150 species in 12 genera (
Spinning fields on spider spinnerets contain the spigots through which silk gland products pass to the external environment. They provide a window to the silk glands, indicating the variety of silk gland types contained within the opisthosoma and their numbers. In mimetids, spigots of major ampullate, piriform, minor ampullate, aciniform, and cylindrical (= tubuliform) silk glands (MaA, PI, MiA, AC, CY, respectively) occupy the spinning fields. The roles of different silk glands have not been studied extensively in this family, but if observations from other araneoids apply, then attachment disks and junctional cements are formed from PI secretions while draglines, bridging lines, prey-swathing fibers, and egg sacs contain MaA silk, MiA silk, AC silk, and CY silk, respectively, and each of these constructions may be supplemented, variably or consistently, by fibers from other silk gland types (
Term | Definition |
---|---|
Proecdysis | The period between apolysis (detachment of epidermis from old exoskeleton in preparation for deposition of new exoskeleton) and ecdysis (shedding of old exoskeleton) |
Tartipore | A minute opening that forms during proecdysis within the developing new exoskeleton around a silk gland duct. It allows the duct to remain connected to a spigot on the old exoskeleton so silk can be drawn from that spigot throughout proecdysis. After ecdysis, the tartipore is non-functional but remains apparent in the exoskeleton. |
Tartipore-accommodated (T-A) silk gland | A silk gland with a duct that remains connected to a spigot on the old exoskeleton throughout proecdysis because an opening (tartipore) forms in the developing new exoskeleton to accommodate the duct. This allows silk to be drawn from the silk gland throughout proecdysis. At ecdysis, the silk gland’s spigot is cast off along with the rest of the old exoskeleton and its duct will not connect to a new spigot until a new exoskeleton forms during the following proecdysis. Thus, these silk glands function in alternate stadia. |
Non-tartipore-accommodated (Non-T-A) silk gland | A silk gland from which silk cannot be drawn during proecdysis because its duct becomes detached from its spigot around apolysis. The duct can, however, then attach to a spigot on the developing new exoskeleton, allowing these silk glands to function in consecutive stadia. |
Primary (1°) ampullate silk glands | Larger ampullate silk glands that are non-T-A. They include 1° major ampullate and 1° minor ampullate silk glands (1° MaA, 1° MiA). |
Secondary (2°) ampullate silk glands | Smaller ampullate silk glands that are T-A. They include 2° major ampullate and 2° minor ampullate silk glands (2° MaA, 2° MiA). |
Nubbin | A vestigial spigot. Three types are recognized: 1) ontogenetic: forms at a site occupied by a functional spigot during an earlier stadium; 2) phylogenetic: forms at a site occupied by a functional spigot in an ancestor; 3) teratological: forms seemingly at random, usually asymmetrically (on one spinneret but not its pair), as a result of a developmental anomaly. |
In earlier studies, spinnerets from five species of Australomimetus were examined by scanning electron microscopy (SEM), all adults (
1° primary
2° secondary
AC aciniform silk gland
AG aggregate silk gland
ALS anterior lateral spinneret
Col colulus
CY cylindrical (= tubuliform) silk gland
FL flagelliform silk gland
MaA major ampullate silk gland
MiA minor ampullate silk gland
MoPI modified piriform silk gland
PI piriform silk gland
PLS posterior lateral spinneret
PMS posterior median spinneret
T-A tartipore-accommodated
Located in the spinning fields of some spinnerets, tartipores form and function only during proecdysis (Table
Only the ducts of some silk glands remain connected to the old exoskeleton during proecdysis and result in tartipores forming in the developing exoskeleton around the ducts. These silk glands are said to be tartipore-accommodated (T-A) and can have silk drawn from them during proecdysis. The ducts of non-tartipore-accommodated (non-T-A) silk glands lose their connection to the old exoskeleton around apolysis, do not give rise to tartipores, and therefore the spider cannot draw silk from them during proecdysis. In some spider clades, such as the ecribellate haplogyne Synspermiata (
Depending on species, sex, spinneret, instar, and maturity, certain spinning fields consistently contain vestigial spigots, termed nubbins, at sites occupied by functional spigots either in earlier instars (ontogenetic nubbins; often present in adults, such as 2° MaA/MiA nubbins) or in ancestors (phylogenetic nubbins). Examples of both these types of nubbins were observed during this study and are noted below. In the literature up through about 1994 (and sometimes beyond), the structures now identified as 2° MaA/MiA tartipores were generally referred to as nubbins [non-vestigial-type nubbins in
With our focus on external features of the exoskeleton, we consider ecdysis to be the event that separates instars (the spiders themselves) and stadia (the periods of time between ecdyses or from ecdysis to death) since, from an external view, it is at ecdysis that there is a change of exoskeleton. Following apolysis, we apply the term pharate instar to only that portion of a proecdysial spider from the new exoskeleton inward. Thus, using the example of a specimen presented extensively in this paper, a 4th instar that undergoes apolysis, considered as a whole, remains a 4th instar throughout proecdysis, and the pharate 5th instar constitutes only a part (albeit the predominant part) of this proecdysial 4th instar. As proposed by
Specimens of Australomimetus spinosus were reared in captivity from egg sacs collected at Yandin-Lookout, Western Australia (30°46’23”S, 115°36’31”E) in August 2014. Australomimetus spinosus is the only mimetid spider that occurs in the Yandin Nature Reserve which comprises open eucalypt woodland and Xanthorrhoea shrubland on rocky outcrops. All egg sacs, containing 10-25 eggs each, were collected from under large rocks and boulders together with females. In the lab, up to four egg sacs were placed in glass jars capped with nylon and the emerging spiderlings were fed with prey spiders (Theridiidae) collected in the field. To avoid cannibalism in A. spinosus post-molt, the spiderlings were progressively separated from the 3rd stadium onwards and 3-5 specimens of later instars were reared in small jars with sufficient prey and some paper tissue. Specimens of all life stages were euthanized in 100% ethanol between August 2014 and January 2015, then stored in a freezer for further analyses. Exuvia of representative specimens were also preserved in ethanol post-molt. Additional museum specimens of A. spinosus were also available from Western Australia, Queensland, and New South Wales, collected between January 1992 and July 2014 and stored in 70-75% ethanol. All specimens of A. djuka were from the collections of the Western Australian Museum (WAM), and stored in 70-75% ethanol. This species has a larger body size but also considerably higher moisture requirements than A. spinosus. It is endemic to the southwest of Western Australia and has been collected from limestone caves in Yanchep National Park but also dense bark accumulations in undisturbed karri (Eucalyptus diversicolor) forest. Additional information on material examined is given in Suppl. material
A total of 11 sets of spinnerets (6 spinnerets/set; 1 set/abdominal exoskeleton) from A. djuka and 63 sets of spinnerets from A. spinosus were examined by SEM (Table
Intact sets of spinnerets, including underlying tissues and small amounts of surrounding exoskeleton, were cut off ethanol-preserved specimens of the two Australomimetus species using Vannas spring scissors (Fine Science Tools, 15000-08) while being viewed through an Olympus SZX12 stereo microscope. They were immersed in Novex® Tris-glycine sodium-dodecyl-sulfate (SDS) buffer (ThermoFisher Scientific, LC2675) at 2X-strength for at least 3 days at 4°C. If underlying tissues did not show signs of partial solubilization in the buffer (expansion, increased translucence, separation from exoskeleton), incubation was continued at room temperature for up to 4 months. After a brief (5 min) rinse in commercial contact lens saline solution, spinnerets were further cleaned, and remaining underlying tissues digested, in Oxysept® Disinfecting Solution containing one Oxysept® Neutralizing Tablet and one Ultrazyme® Enzymatic Cleaner Tablet (all Abbott Medical Optics, intended for use with soft contact lenses) according to manufacturer’s instructions. This treatment exposed spinnerets to 3% H2O2 and subtilisin A and was allowed to proceed until underlying tissues were no longer visible, occasionally requiring overnight incubation. Spinnerets were then transferred to a Petri dish containing a wax layer overlaid with the above Novex® buffer. The tips of pins of various sizes projected 1-3 mm upward from the wax layer (
Some 2nd and 3rd stadia spinnerets were obtained from exuvia that were initially stored in 70-75% ethanol, but were subsequently immersed in the aforementioned SDS-containing Novex® buffer for at least 2 months at 4°C. These were prepared like the above spinnerets from intact specimens except that they were not treated with the H2O2/subtilisin A solution and they were not intentionally separated from the rest of the exuvium, though separation often resulted nonetheless from the act of pulling the spinnerets down onto the tips of pins to expand them.
Diameters of AC spigot openings, presented in Figs
Copulatory structures, or evidence of their development, were the primary features consulted in determining the sex of adults and penultimate instars. The sex of younger specimens, as early as 3rd instars of A. spinosus and 4th instars of A. djuka, was primarily determined by the presence (female) or absence (male) of CY spigots on PMS and PLS. As detailed in the Results (‘Cylindrical silk gland…’), the full complement of CY spigots was not present in the majority of 3rd stadium female A. spinosus, raising the possibility that some 3rd stadium females do not exhibit any CY spigots and leaving some uncertainty in our assignment of sex for a few 3rd stadium individuals. CY spigots appear to be absent in female as well as male 1st and 2nd instars. Thus, sex was not determined for these instars.
Convention 1. Each of the three pairs of spinnerets (ALS, PMS, PLS), split midsagittally, form right/left mirror-images (in overall arrangement) of each other. Some spinnerets depicted in the figures were from the right side of the opisthosoma, some from the left. But to facilitate comparisons, those from the right side have been flipped in Microsoft® PowerPoint® 2016 so that they also appear to be left spinnerets. Figure legends, however, state their true handedness (right, left). Figs
Convention 2. On each anterior lateral spinneret (ALS) and posterior lateral spinneret (PLS), spinning fields are restricted to the more distal of the two segments composing the spinneret. Posterior median spinnerets (PMS) are single-segmented with spigots apically placed. Fig.
Spinnerets of Australomimetus spinosus. A. Overview of spinnerets. Boxes indicate approximate regions and perspectives shown in many of the images in Figs
ALS of 1st to 3rd instars of Australomimetus spinosus and Australomimetus djuka. A–C. A. spinosus. D–F. A. djuka. A, D. Boxed regions, magnified in insets, show putative 2° MaA tartipore primordia. Unlabeled arrows indicate the four PI spigots present in 1st instars (two T-A, two non-T-A); note increased PI spigot numbers in later instars. B, E. Unlabeled arrows indicate the two PI tartipores present in 2nd instars; note increased PI tartipore numbers in later instars. A, C, E, F. Left ALS. B, D. Right ALS (image flipped). 1°, 1° MaA spigot; 2°, 2° MaA spigot; T, 2° MaA tartipore. All scale bars 20 µm. See also Abbreviations and terminology (‘Spinning apparatus abbreviations’) and Results (Convention 3 in ‘Conventions applied…’).
Convention 3. Figures include a label indicating the instar from which the spinneret shown was obtained (1st to 6th; Ph, pharate instar) and, if known, the spider’s sex (♀, ♂) followed by a state of maturity subscript (J, juvenile; A, adult). For example, 4th ♀J indicates a juvenile female 4th instar. Sex was unknown only for 1st and 2nd instars, though uncertain for some 3rd instars [see Results (‘Cylindrical silk gland…’)].
Convention 4. One of the new findings of the present study is that A. spinosus, unlike A. djuka and five other Australomimetus species previously examined (
Convention 5. As noted in ‘Spinnerets examined’ in Materials and methods, in only one instance were two consecutive sets of spinnerets from the same individual identified; 4th and pharate 5th instar sets from a proecdysial 4th stadium juvenile male A. spinosus, separated from one another during enzyme cleaning and examined separately. All of the colorized micrographs (Figs
ALS of 4th to 6th instars of Australomimetus spinosus. A-C.ALS from one juvenile specimen, well into proecdysis. Color coding explained in Results (Convention 5 in ‘Conventions applied…’). A. Old exoskeleton (all spigots but one damaged; 1° MaA spigot torn out entirely, but its former location is colored red). B. New exoskeleton directly beneath (A). C. New exoskeleton, opposite ALS from (A, B), demonstrating utility of a tartipore. Unlabeled arrow to 2° MaA duct (severed at distal end) still accommodated by 2° MaA tartipore (T). Before old and new exoskeletons were separated, this duct emptied on a 2° MaA spigot on old exoskeleton (like that in (A) but on opposite ALS). D-G. Adults. Presumed pair of non-T-API spigots (stars) on each ALS in males (D, E) were morphologically similar to T-API spigots (unlabeled PI spigots) in both sexes and to non-T-API spigots (stars) in females (F, G): no MoPI spigots. C. Left ALS. A, B, D-G. Right ALS (image flipped). 1°, 1° MaA spigot; 2°, 2° MaA spigot; N, 2° MaA nubbin (ontogenetic); T, 2° MaA tartipore. Scale bars: A-D, F 20 µm; E, G 5 µm. See also Abbreviations and terminology (‘Spinning apparatus abbreviations’) and Results (Convention 3 in ‘Conventions applied…’).
Red: Spigots of non-T-A silk glands that were functional during the 4th stadium and would have been functional again during the 5th and 6th (adult) stadia, though not during proecdyses. Thus, they were not functional at the time the spider was preserved.
Purple (Fuchsia): Spigots of non-T-A silk glands that would have been functional for the first time during the 5th stadium, though not during proecdysis, and then again during the 6th (adult) stadium. Only certain AC spigots (Fig.
Blue: Spigots and tartipores of T-A silk glands that were functional during the 4th stadium, at least during proecdysis (and presumably also well prior to apolysis for at least some of these silk glands, such as PI). The PI and AC members of this group would have been functional again during the 6th (adult) stadium, while the 2° MaA and 2° MiA members would have atrophied, their spigots replaced by nubbins. In the 5th instar, the presence of these silk glands would have been indicated externally only by post-functional tartipores.
Yellow: Spigots and tartipores of T-A silk glands that would have been functional during the 5th stadium and had previously been functional during the 3rd stadium, at least during proecdyses (and presumably also well prior to apolysis for at least some of these silk glands, such as PI). None of these would have been functional during the 6th (adult) stadium. In the 4th instar, the presence of these silk glands was indicated externally only by post-functional tartipores.
Green: Spigots of T-A silk glands that would have been functional for the first time during the 5th stadium, at least during proecdysis (and presumably also well prior to apolysis for at least some of these silk glands, such as PI). None of these would have been functional during the 6th (adult) stadium.
Convention 6. Tables
Spigot, tartipore, and nubbin complements on spinnerets of Australomimetus djuka and Australomimetus spinosus.
Instar | Species | Sex | N b | ALS | PMS | PLS | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1° MaA spigot | 2° MaA spigot | 2° MaA tartiporec | 2° MaA nubbin | PI spigots | PI tartipores | 1° MiA spigot | 2° MiA spigot | 2° MiA tartiporec | 2° MiA nubbin | AC spigots | AC tartipores | CY spigot | AC spigots | AC tartipores | CY spigot | ||||
1st | Australomimetus djuka | unknown | 2 | 1 | 1 | 1 prim. | 0 | 4 | 0 | 1 | 0 | 0 | 1 | 1 | 0 | 0 | 2 | 0 | 0 |
1st | Australomimetus spinosus | unknown | 6 | 1 | 1 | 1 prim. | 0 | 4 | 0 | 1 | 1 | 1 prim. (AL) | 0 | 2 | 0 | 0 | 3 | 0 | 0 |
2nd | Australomimetus djuka | unknown | 1 | 1 | 1 | 1 | 0 | 7.5 (7-8) | 2 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 2 | 0 | 0 |
2nd | Australomimetus spinosus | unknown | 13 | 1 | 1 | 1 | 0 | 8.2 ± 0.12 (7-10) | 2 | 1 | 1 | 1 (PM) | 0 | 2 | 0 | 0 | 3.1 ± 0.08 (3-4) | 0 | 0 |
3rd | Australomimetus djuka | unknown | 1 | 1 | 1 | 1 | 0 | 12.0 (11-13) | 4 | 1 | 0 | 0 | 0 | 2 | 0 | 0 | 3 | 0 | 0 |
3rd | Australomimetus spinosus | ♀ | 5 | 1 | 1 | 1 | 0 | 12.3 ± 0.34 (11-13) | 5.9 ± 0.19 (5-7) | 1 | 1 | 1 (AL) | 0 | 2 | 0 | 0.40 ± 0.187 (0-1) | 4.7 ± 0.12 (4-5) | 0.10 ± 0.100 (0-1) | 0.90 ± 0.100 (0-1) |
3rd | Australomimetus spinosus | ♂ | 10 | 1 | 1 | 1 | 0 | 12.5 ± 0.24 (11-14) | 6.5 ± 0.16 (6-8) | 1 | 1 | 1 (AL) | 0 | 2 | 0 | 0 | 5.3 ± 0.11 (4-6) | 0.05 ± 0.050 (0-1) | 0 |
4th | Australomimetus djuka | ♀ | 1 | 1 | 1 | 1 | 0 | 21.5 (21-22) | 13 | 1 | 0 | 0 | 0 | 2 | 0 | 1 | 4.5 (4-5) | 0 | 1 |
4th | Australomimetus spinosus | ♀ | 5 | 1 | 1 | 1 | 0 | 18.3 ± 0.34 (16-19) | 10.3 ± 0.25 (9-11) | 1 | 1 | 1 (PM) | 0 | 2 | 0 | 1 | 7.8 ± 0.25 (7-9) | 0.90 ± 0.245 (0-2) | 1 |
4th | Australomimetus spinosus | ♂ | 8 | 1 | 1 | 1 | 0 | 18.6 ± 1.00 (16-25) | 10.9 ± 0.55 (9-14) | 1 | 1 | 1 (PM) | 0 | 2.1 ± 0.13 (2-3) | 0 | 0 | 7.6 ± 0.22 (6-9) | 1.5 ± 0.21 (0-3) | 0 |
5th Juvenile | Australomimetus djuka | ♀ | 3 | 1 | 1 | 1 | 0 | 32.5 ± 1.53 (28-35) | 19.7 ± 3.18 (16-26) | 1 | 0 | 0 | 0 | 1.8 ± 0.17 (1-2) | 0 | 1 | 5.8 ± 0.44 (5-7) | 0 | 1 |
5th Adult | Australomimetus djuka | ♂ | 2 | 1 | 0 | 1 | 1 | 30.0 ± 1.50 (28-33) | 20.3 ± 1.25 (19-22) | 1 | 0 | 0 | 0 | 2.5 ± 0.50 (2-3) | 0 | 0 | 5.5 ± 0.50 (5-6) | 0 | 0 |
5th Juvenile | Australomimetus spinosus | ♀ | 3 | 1 | 1 | 1 | 0 | 27.0 ± 2.57 (22-33) | 16.0 ± 1.00 (14-18) | 1 | 1 | 1 (AL) | 0 | 2 | 0 | 1 | 9.2 ± 0.67 (8-11) | 2.7 ± 0.73 (1-4) | 1 |
5th Juvenile | Australomimetus spinosus | ♂ | 4 | 1 | 1 | 1 | 0 | 25.1 ± 1.07 (22-29) | 15.1 ± 1.30 (12-18) | 1 | 1 | 1 (AL) | 0 | 2 | 0 | 0 | 9.9 ± 0.13 (9-11) | 2.6 ± 0.24 (2-3) | 0 |
5th Adult | Australomimetus spinosus | ♀ | 2 | 1 | 0 | 1 | 1 | 42.8 ± 3.25 (39-46) | 21 | 1 | 0 | 1 (AL) | 1 | 2.3 ± 0.25 (2-3) | 0 | 1 | 12.0 ± 1.00 (11-13) | 4.3 ± 0.25 (4-5) | 1 |
5th Adult | Australomimetus spinosus | ♂ | 3 | 1 | 0 | 1 | 1 | 31.5 ± 1.04 (28-33) | 19.0 ± 1.32 (16-22) | 1 | 0 | 1 (AL) | 1 | 3.7 ± 0.33 (3-4) | 0 | 0 | 11.0 ± 0.58 (10-12) | 4.2 ± 0.83 (2-5) | 0 |
6th Adult | Australomimetus djuka | ♀ | 1 | 1 | 0 | 1 | 1 | 45.5 (44-47) | 35.5 (35-36) | 1 | 0 | 0 | 0 | 2 | 0 | 1 | 7.5 (7-8) | 0 | 1 |
6th Adult | Australomimetus spinosus | ♀ | 2 | 1 | 0 | 1 | 1 | 42.5 ± 0.50 (41-44) | 25.5 ± 1.00 (23-27) | 1 | 0 | 1 (PM) | 1 | 2 | 0 | 1 | 12.3 ± 0.25 (12-13) | 4.5 ± 0.50 (4-5) | 1 |
6th Adult | Australomimetus spinosus | ♂ | 2 | 1 | 0 | 1 | 1 | 35.5 ± 3.00 (32-40) | 21.5 ± 2.50 (19-25) | 1 | 0 | 1 (PM) | 1 | 4.3 ± 0.25 (4-5) | 0 | 0 | 13.8 ± 0.25 (13-14) | 5.3 ± 0.25 (5-6) | 0 |
Preserved spiders and exuvia were assigned to specific instars/stadia based on numbers of spigots and tartipores from the three pairs of spinnerets and, for A. spinosus, the position of the 2° MiA tartipore (Table
Based on examination of spinnerets from nine adults, both female and male A. spinosus have the potential to become adults after either five or six molts [Table
Contrary to what was observed on the other two pairs of spinnerets, described below, there were no substantial differences between ALS of A. spinosus and A. djuka. Both had single 1° and 2° MaA spigots on each ALS in all instars, with the exception that the 2° MaA spigot was replaced by a 2° MaA nubbin in adults (Table
ALS of 4th to 6th instars of Australomimetus djuka. A. Juvenile. B–E. Adults. Presumed pair of non-T-API spigots (stars) on each ALS in males (B, C) were morphologically similar to T-API spigots (unlabeled PI spigots) in both sexes and to non-T-API spigots (stars) in females (D, E): no MoPI spigots. A, D, E. Left ALS. B, C. Right ALS (image flipped). 1°, 1° MaA spigot; 2°, 2° MaA spigot; N, 2° MaA nubbin (ontogenetic); T, 2° MaA tartipore. Scale bars: A, B, D 20 µm; C, E 10 µm. See also Abbreviations and terminology (‘Spinning apparatus abbreviations’) and Results (Convention 3 in ‘Conventions applied…’).
When PI spigot numbers in specimens at one stadium were compared with PI tartipore numbers in specimens at the next stadium, there were further indications of a constant two non-T-API/ALS in all instars. For example, in A. spinosus, PI spigots in 13 2nd instars and PI tartipores in 15 3rd instars numbered from 7-10 (Ῡ = 8.2)/ALS and 5-8 (Ῡ = 6.3)/ALS, respectively (Table
In the proecdysial 4th stadium male A. spinosus, nine PI tartipores were present in the old exoskeleton (Fig.
In Fig.
Postembryos were not examined, but all indications were that functional spigots first appeared in 1st instars. Thus, in both species, PI and 2° MaA tartipores first appeared in the exoskeletons of 2nd instars (Fig.
In some mimetid males, maturity brings the appearance of a pair of modified PI (MoPI) spigots to each ALS that differ morphologically from the other PI (
PMS of the two species differed from the 1st stadium. In A. djuka, 1st and 2nd instars had one AC spigot/PMS, increasing to two in a 3rd instar, while A. spinosus invariably started off with two AC spigots/PMS in 1st instars (Table
PMS of 1st to 3rd instars of Australomimetus spinosus and Australomimetus djuka. All unlabeled arrows point to non-T-AAC spigots. A–D. A. spinosus. A. Boxed region, magnified in inset, shows putative 2° MiA tartipore primordium. B–D. 2° MiA spigot (2°) and 2° MiA tartipore (T) switched positions from 2nd to 3rd stadium. This switching continued in later stadia (Fig.
The absence of functional 2° MiA in all examined specimens of A. djuka appears to represent a secondary loss: an apparent phylogenetic 2° MiA nubbin was observed on all four examined PMS from two 1st instars of A. djuka (Table
On both old and new exoskeletons from the proecdysial 4th stadium male A. spinosus [see Results (Convention 5 in ‘Conventions applied…’)], spigots of non-T-A silk glands on each PMS included all (two) AC spigots and the 1° MiA spigot (Fig.
PMS of 4th to 6th instars of Australomimetus spinosus. All unlabeled arrows point to non-T-AAC spigots. A, B.PMS from one juvenile specimen, well into proecdysis. Color coding explained in Results (Convention 5 in ‘Conventions applied…’). A. Old exoskeleton. B. New exoskeleton directly beneath (A). C. Adult female. With one exception [see Results (‘PMS-Sexual and geographic…’) and Discussion (‘AC spigot numbers…’)], adult females had two AC spigots/PMS. D-G. Adult males from eastern Australia (New South Wales) (D, E) and Western Australia (F, G). AC spigots magnified in (E, G) to show wider openings on ‘late’ AC spigots (L) relative to neighboring ‘pioneer’ AC spigots (P) [see Results (‘PMS-Sexual and geographic…’)]; measured diameters of openings given in nm [see Materials and methods (‘SEM of spinnerets…’)]. In (G), distal ends of AC spigot shafts magnified in insets (connected by dashed lines), with each spigot’s counterpart on the opposite PMS shown in an adjacent inset. H–J. Ontogenetic changes in CY spigot (C) morphology; lateral view. Note changes in width relative to nearest AC spigot (arrow). A–G, I, J. Left PMS. H. Right PMS (image flipped). 1°, 1° MiA spigot; 2°, 2° MiA spigot; C, CY spigot; L, ‘late’ AC spigot; N, 2° MiA nubbin (ontogenetic); P, ‘pioneer’ AC spigot; T, 2° MiA tartipore. Scale bars: A-D, F 20 µm; E, G-J 10 µm. See also Abbreviations and terminology (‘Spinning apparatus abbreviations’) and Results (Convention 3 in ‘Conventions applied…’).
AC associated with the PMS appeared to present an example of sexual dimorphism in A. spinosus, though examination of additional specimens will be needed to establish the consistency of certain details. With one exception, the number of AC spigots in females was a constant two/PMS at all stadia (Table
Aciniform silk gland (AC) occurrence on posterior spinnerets (PMS + PLS) of Australomimetus spinosus.
Instar | Sex | Australian Regionc | N d | PMS b | PLS | ||||
---|---|---|---|---|---|---|---|---|---|
Non-T-A | AnterioreT-A | Non-T-A | PosterioreT-A | ||||||
AC spigots | AC spigots | AC tartipores | AC spigots | AC spigots | AC tartipores | ||||
1st | unknown | West | 6 | 2 | 0 | 0 | 3 | 0 | 0 |
2nd | unknown | West | 13 | 2 | 0 | 0 | 3 | 0.12 ± 0.083 (0-1) | 0 |
3rd | ♀ | West | 5 | 2 | 0.10 ± 0.100 (0-1) | 0 | 4 | 0.60 ± 0.187 (0-1) | 0.10 ± 0.100 (0-1) |
3rd | ♂ | West | 10 | 2 | 0.30 ± 0.082 (0-1) | 0 | 4 | 0.95 ± 0.050 (0-1) | 0.05 ± 0.050 (0-1) |
4th | ♀ | West | 5 | 2 | 0.90 ± 0.100 (0-1) | 0.20 ± 0.122 (0-1) | 5 | 1.9 ± 0.24 (1-3) | 0.70 ± 0.122 (0-1) |
4th | ♂ | West | 7 | 2 | 0.93 ± 0.071 (0-1) | 0.43 ± 0.130 (0-1) | 5 | 1.4 ± 0.07 (1-2) | 0.93 ± 0.071 (0-1) |
4th | ♂ | East | 1 | 3 | 2 | 1 | 5 | 2 | 1.5 (1-2) |
5th Juvenile | ♀ | West | 2 | 2 | 1.3 ± 0.25 (1-2) | 0.75 ± 0.250 (0-1) | 6 | 1.3 ± 0.25 (1-2) | 1.3 ± 0.25 (1-2) |
5th Juvenile | ♂ | West | 4 | 2 | 1.8 ± 0.14 (1-2) | 1 | 6 | 2.1 ± 0.13 (2-3) | 1.6 ± 0.24 (1-2) |
5th Adult | ♂ | West | 1 | 3 | 2 | 1 | 6 | 2 | 1.5 (1-2) |
5th Juvenile | ♀ | East | 1 | 2 | 2 | 2 | 6 | 2.5 (2-3) | 2 |
5th Adult | ♀ | East | 2 | 2.3 ± 0.25 (2-3) | 2.5 ± 0.50 (2-3) | 2 | 6 | 3.5 ± 0.50 (3-4) | 2.3 ± 0.25 (2-3) |
5th Adult | ♂ | East | 2 | 4 | 2.5 ± 0.50 (2-3) | 2 | 6 | 3 | 3 |
6th Adult | ♀ | West | 1 | 2 | 2 | 1 | 7 | 3 | 3 |
6th Adult | ♀ | East | 1 | 2 | 3 | 2 | 7 | 2.5 (2-3) | 3 |
6th Adult | ♂ | East | 2 | 4.3 ± 0.25 (4-5) | 3.3 ± 0.25 (3-4) | 2 | 7 | 3.5 ± 0.00 (3-4) | 3.3 ± 0.25 (3-4) |
AC spigot data from A. djuka did not suggest a clear sexual dimorphism like that indicated in A. spinosus, though there were some similarities. Females, including the single adult female examined, usually had two AC spigots/PMS and never exceeded this number (Table
PMS of 4th to 6th instars of Australomimetus djuka. All unlabeled arrows point to non-T-AAC spigots. A, B. Juveniles. C, D. Adults. Same CY spigot (C) and closest AC spigot (unlabeled arrow) shown from two perspectives in (A, C); inset in (B) likewise from same specimen used to produce larger image, but shows lateral view of CY and AC spigots from opposite PMS. Note ontogenetic changes in CY spigot width relative to nearest AC spigot. E.AC spigots from (D) magnified, the distal ends of their shafts further magnified in insets: no obvious difference in diameters of openings, given in nm [see Materials and methods (‘SEM of spinnerets…’)], between ‘late’ AC spigot (L) and ‘pioneer’ AC spigots (P) [see Results (‘PMS-Sexual and geographic…’)]. A, B inset, C. Left PMS. B main, D, E. Right PMS (image flipped). 1°, 1° MiA spigot; C, CY spigot; L, ‘late’ AC spigot; P, ‘pioneer’ AC spigot. Scale bars: A-D 20 µm; E 10 µm. See also Abbreviations and terminology (‘Spinning apparatus abbreviations’) and Results (Convention 3 in ‘Conventions applied…’).
The PMSAC data also suggested a difference among males of A. spinosus by geography. Again, it will be necessary to examine more specimens to better evaluate this difference, but the data obtained thus far were consistent in indicating that late stadium males from eastern Australia have more AC spigots/PMS than males from Western Australia (Table
Observations pertaining to PMSCY are presented below.
As with PMS-associated AC, the two species differed during the earliest stadia with respect to the number of non-T-AAC spigots/PLS: two in 1st and 2nd instars of A. djuka (Fig.
PLS of 1st to 3rd instars of Australomimetus djuka and 1st to 4th instars of Australomimetus spinosus. A–C. A. djuka. D–L. A. spinosus. All spigots in (A–E, G) are non-T-AAC spigots. AC spigots between two white curves in (F, H, I, L) are non-T-A; remainder are T-A. E, F.AC spigot complements in 2nd instars of A. spinosus: three non-T-A (E) (88%, N = 26 PLS); three non-T-A, one posterior T-A (F) (12%). G–I.AC spigot complements in 3rd instars of A. spinosus: four non-T-A (G) (13%, N = 30 PLS); four non-T-A, one posterior T-A (H) (63%); four non-T-A, one anterior T-A (not shown) (3%); four non-T-A, one posterior T-A, one anterior T-A (I) (20%). One posterior AC tartipore (G, magnified in inset) observed in 7% of 3rd stadium PLS; the rest had none. H.CY spigots (C) first appeared in 3rd instars of A. spinosus. J, K.AC spigots on PLS from different A. spinosus specimens, showing larger diameter openings on T-AAC spigots (arrows) compared with neighboring non-T-AAC spigots [see Results (‘PLS’)]; measurements given in nm [see Materials and methods (‘SEM of spinnerets…’)]. L.PLS containing possible phylogenetic nubbin, adjacent to putative sensillum (S), in position consistent with araneoid AG-FL spigots. Lower inset, same nubbin (N) and sensillum magnified, anterior view; upper inset, sensillum from opposite PLS, lacking a nubbin; circles indicate (indistinct) raised-rim pores that are part of sensilla. More distinct pores can be seen in
Most 2nd stadium PLS of A. spinosus (23 of 26 PLS from 13 2nd instars) had the same spinning complement as 1st stadium PLS: three non-T-AAC spigots (Fig.
Consistent with the prevalence of one or two T-AAC spigots on 3rd stadium PLS, one (Fig.
PLS of 4th to 6th instars of Australomimetus spinosus. A–D.PLS from one juvenile specimen, well into proecdysis. Color coding explained in Results (Convention 5 in ‘Conventions applied…’). A, B. Old exoskeleton. C, D. New exoskeleton directly beneath (A, B), respectively. One of two AC tartipores (yellow) in (B) largely obscured (unlabeled arrow). E, F. Adult, AC spigots between two white curves are non-T-A; the rest, anterior and posterior, are T-A. AC spigots in boxed region of (E) magnified in (F), showing larger diameter openings in T-AAC spigots compared with neighboring non-T-AAC spigots [see Results (‘PLS’)]; measurements given in nm [see Materials and methods (‘SEM of spinnerets…’)]. G-I. Ontogenetic changes in CY spigot (C) morphology; medial, high tilt view. Note changes in width relative to AC spigot on right. (E, F, I) all from same PLS. B, D–F, I. Left PLS. A, C, G, H. Right PLS (image flipped). C, CY spigot; S, putative sensillum. Scale bars: A-E 20 µm; F-I 10 µm. See also Abbreviations and terminology (‘Spinning apparatus abbreviations’) and Results (Convention 3 in ‘Conventions applied…’).
Positions of T-AAC spigots and tartipores, relative to non-T-AAC spigots, in Fig.
Morphological differences between T-A and non-T-AAC spigots in A. spinosus were modest: the former sometimes had narrower bases than the latter, especially during earlier stadia (e.g., Fig.
Geographical differences between eastern and western populations of A. spinosus with respect to PLS were also modest and, as with geographical differences noted above for PMSAC, will need to be confirmed or negated by examining more material. The data currently available indicate that, from as early as the 3rd stadium (based on AC tartipore counts in 4th instars), eastern A. spinosus have higher mean counts of T-AAC than their western counterparts (Table
No spigots of aggregate or flagelliform silk glands (AG, FL) were observed on any of the 22 A. djukaPLS and 126 A. spinosusPLS that were examined. However, a possible phylogenetic nubbin was present on the right PLS of one 4th stadium female A. spinosus, in a position consistent with the AG-FL triad of typical araneoids (Fig.
Observations pertaining to PLSCY are presented below.
PLS of 4th to 6th instars of Australomimetus djuka. All spigots other than CY spigots (C) are non-T-AAC spigots. A–C. Juveniles. D–F. Adults. B–E. Unlike A. spinosus, variation observed in non-T-AAC spigot number within same stadium and state of maturity. CY spigot and nearby AC spigots shown from two perspectives in (A, C, F). B, C, E, F. Left PLS. A, D. Right PLS (image flipped). C, CY spigot; S, putative sensillum. All scale bars 20 µm. See also Abbreviations and terminology (‘Spinning apparatus abbreviations’) and Results (Convention 3 in ‘Conventions applied…’).
In both species, a full complement of CY spigots, restricted to females, consisted of four CY spigots; one on each PMS and PLS.
In A. spinosus, CY spigots were first seen in 3rd instars (Figs
In A. djuka, because of the small number of early juveniles examined, we can only say that CY spigots are present at least by the 4th stadium: the only 4th instar examined had the full complement of CY spigots (Figs
In both species, increases in widths of CY spigots (bases and shafts) over successive stadia were disproportionately large compared with increases in nearby AC spigots [cf. Figs
The presence of PLSAC tartipores in A. spinosus (Fig.
Another variable character among different species of Australomimetus is the presence or absence of 2° MiA, as revealed by the presence/absence of a 2° MiA tartipore and either a 2° MiA spigot (juveniles) or an ontogenetic 2° MiA nubbin (adults) on each PMS. Previously, only one of the five aforementioned Australomimetus species was found to lack 2° MiA: A. tasmaniensis. This study clearly places A. spinosus in the larger contingent possessing 2° MiA (Figs
Differences among species of Australomimetus with respect to these two characters, PLST-AAC and 2° MiA, suggest that some species have greater silk needs during proecdysis, and ecdysis (
Figures
Further evidence came from a 4th instar male A. spinosus that was well into proecdysis when it was preserved [see Results (Convention 5 in ‘Conventions applied…’)], allowing us to examine spinnerets on both the old (4th instar) and newly developed (pharate 5th instar) exoskeletons. This specimen was the source of all colorized images in Figs
To date, a pair of non-T-API associated with each ALS have only been indicated in three araneoid genera: Araneus, Mimetus, and now Australomimetus. In contrast, in the lycosids that have so far been examined, all PI are T-A (
Dissections of ecribellate orb-web builders, especially Araneus, at various points in the molt-intermolt cycle, have demonstrated that 1° ampullate silk glands (1° MaA and 1° MiA), being non-T-A, are re-used during each successive stadium (
Because ampullate silk glands are relatively large, and few and constant in number across juvenile stadia, gross morphological changes can be monitored by dissection over the molt-intermolt cycle. PI and AC, on the other hand, generally occur as morphological multiples (
In the lycosids they studied,
In A. spinosus, conserved arrangements of AC spigots on the PLS during development largely mirrored those of PI spigots on the ALS. Thus, AC tartipores on the old exoskeleton in Fig.
Though a given T-AAC or PI functions only during alternate stadia, there are indications, particularly from the PLST-AAC in A. spinosus, that these silk glands have some latitude with respect to when they “enter the work force”. That is, an individually specifiable T-AAC may function during odd-numbered stadia in some specimens, but function during even-numbered stadia in other specimens. Consider the following: most (88%, N = 26 PLS) 2nd stadium PLS had no T-AAC spigots (Fig.
Adult males of A. spinosus and A. djuka did not exhibit a pair of spigots on each ALS serving modified PI (MoPI). Such spigots, located near the 2° MaA tartipore, have been found exclusively in adult males of some mimetid species. They are conspicuously different from other PI spigots, with wider shafts and openings, and segregated from the other PI spigots. It is unknown what purpose MoPI serve, but their restriction to adult males advocates for investigation of silken structures specific to courtship and mating (
MoPI spigots were first described in two North American and one South American species of Mimetus (
MoPI spigots like those in Mimetus, Phobetinus, and Gelanor have not been evident in those Ero or Australomimetus species examined to date (
CY, also known as tubuliform silk glands, are used by adult females in the construction of egg sacs. The two Australomimetus species examined in this study exhibited the unusually large, wide-aperture CY spigots typical of adult female mimetines (
The deep incising typical of CY spigot shafts in Ero and Mimetus (
Remarkably, though the newly described mimetid genus Anansi nests within the subfamily Mimetinae, scans of their spinnerets (
With the exception of a single PMS, there were two AC spigots on each PMS in four adult female A. spinosus. In contrast, five adult males had 3-5 AC spigots on each PMS and one juvenile male from eastern Australia had three per PMS (Table
The specimens of A. spinosus examined in this study included examples from Western Australia and from the eastern states of Queensland and New South Wales. Only slight differences in spinneret features were indicated between these populations, and because of the limited data, additional observations will be required to verify or refute them. These differences concerned the (never high) numbers of AC spigots, with eastern specimens tending to have more T-AAC spigots on PLS and, among older males, more non-T-AAC spigots on PMS (Table
In most, if not all, araneomorphs, functional T-A silk glands are first available for use in 1st instars (
The position taken by the structure in question on PMS of 1st instars of A. spinosus was also consistent with a tartipore primordium identity. Relative to the 2° MiA spigot, its location varied over about a 90° arc on different 1st stadium PMS, from directly anterior to the spigot (as in Fig.
Like other Mimetidae (
We are immensely grateful to collectors of some of the specimens examined in this study: G.J. Anderson, C.J. Burwell, R. Foulds, R. Graham, M. Gray, S.M. Harms, M.S. Harvey, W.F. Humphreys, G. Milledge, A. Nakamura, J. Norman, J. Peck, S. Peck, C. Rippon, H. Smith, and J.M. Waldock. We are also thankful to M.S. Harvey and Julianne Waldock (Western Australian Museum), Robert Raven and Owen Seeman (Queensland Museum), and Graham Milledge (Australian Museum) for the loan of specimens from their collections, and to Petr Dolejš (National Museum-Natural History Museum, Praha) and an anonymous reviewer for many helpful comments that improved the manuscript. Stephanie Harms assisted with the collection of prey spiders for the hungry pack and raising several hundred pirate spider babies would have been impossible without her fierce efforts. Charlene Newton very generously assisted with the creation of Fig.
Material Examined
Data type: specimens data