Larinioides Orbweaver (adult male + adolescent/juvenile females)
Lake Lavon, TX 08-09 Jul 2009, Kendra N.
Thanks again to Kendra for these excellent shots, taken with a Sony digital camera on the balcony of her apartment near lake Lavon, Texas. After she sent photographs of the adult female Larinioides orbweaver on 07 Jul 2009, I asked her to try to get ventral photos of that spider. The next night, and again the following evening, she took additional photos that were found to include what appears to be an adult male, an adolescent female, and a juvenile female of the same species of spider.
The photo above portrays the ventrum (underside) of what appears to be an adult male that is smaller than the adult female. Orbweavers are, as a rule, sexually dimorphic. That is, the females and males differ dramatically in size, with the males being smaller. For some, e.g. the yellow garden spider (Argiope aurantia Lucas) the size difference often exceeds 50:1 dimensionally, and may exceed 1000:1 in weight. As a result, the males and females appear, to the uninitiated, as quite unrelated species. In others, e.g., the western spotted orbweaver (Neoscona oaxacensis), and the present species in the genus Larinioides, the differences between the sexual forms is less, so that the male merely appears as a somewhat undersized female.
We dispel that misapprehension by making a closer examination of the genitalia. Specifically, in the space between the book lung slits of an entelegyne species (the Araneidae, or orbweavers, are all entelegyne spiders that typically possess conspicuous external genitalia) we expect to see a three-dimensional, sclerotized structure that marks the exterior of the female's epigynum. In this specimen, however, we find only smooth, apparently unbroken skin. That, of course, is probably illusory, as the male's sperm ducts would be here here, but their external orifice is microscopic and would not be expected to show in this photograph. On the other hand, in the early stages of nymphal development, the female's genital structures are either rudimentary or absent. This could be a nymph. So we look further.
Besides lacking anything suggestive of an epigynum in the expected place on the ventral abdomen, this specimen's palps (the two diminutive, leg-like structures extending from each side of the head) are swollen at their distal extremities. In the entelegyne female, the palps are not enlarged at their tips but merely appear as diminutive legs whose palpal tarsi are, as with ordinary legs, the smallest segments of that extremity. Not so are those of an entelegyne male. Instead of terminating in tarsi, the male palp terminates in a complicated structure that is conspicuously larger than the preceding, proximal segments. These are the emboli, which the male charges with sperm prior to mating, in an instinctual process that begins with the construction of a sperm web, on which is deposited a drop of sperm which, as Rainer F. Foelix puts it in his book "Biology of Spiders" (2nd Ed., 1996, p. 177) the male exudes from a ventral opening--the epigastric furrow--between the book lungs. This drop is then laboriously worked into each palpal embolus. As the above enlargement shows, this specimen sports an enlarged palpal terminus. We cannot discern, from this photograph, the intricacies of those structures beyond the fact that they appear swollen, but it is enough that they do not resemble mere palpal tarsi. We may now assert with some confidence, then, that this is a male.
But, is it a male of the same species as the female shown in Kendra's previous photos? Some would hastily affirm that to be a safe assumption, one in fact that needs no further proof, simply because a male would have no reason to loiter here otherwise. In fact, to do so would invite the nearby females to make a quick meal of him. Of course, the same general threat hangs over the males of the same species, but the latter possess, besides the right chemotactic qualities and associated pheromonal attributes, a repertoire of instinctive behavioral advantages that enable them to remain alive long enough to mate. Unrelated males, by comparison, would likely last no more than a few minutes or hours. Shall we, then, assume this is male of the species, or might it be, instead, a foolish interloper of another sort whose presence is a temporary fluke?
As any experienced skeptic knows--usually from painful mistakes of the past--assumptions in scientific pursuits often produce inexcusable errors. One should avoid the inexcusable, methinks. We therefore proceed to amass further evidence.
Our search would be assisted if we could compare various anatomical features of each specimen, side by side. One must, of course, exploit whatever one is given to its full advantage, but in this case such a comparison is hampered by the fact that none of Kendra's previous photographs show the adult female in a pose that is even roughly analogous to that taken by this male. Yet we do have an adolescent female--whose features compare so favorably with those of the adult that there seems little doubt they are the same species--posed in a manner similar to that of the male, albeit dorsally rather than ventrally. Though Kendra worked hard to get us the best photos she could, the adult female refused to cooperate by providing a ventral view.
Upon placing the ventral male and dorsal adolescent female poses side by side, in the above composite, we see that the colorations of the various leg segments match rather well, and that the setae on each leg seem a close match, too. Note that the third leg--L3, counting from the most forward leg (L1) backward--is the shortest in both specimens. But notice, too, that the patella of legs 1, 2, and 4 is elongated in the juvenile female (depicted in the right half of the photo), compared to that of the male. I find that particularly interesting, because one would expect the opposite. The legs of the male are generally longer in most aspects than those of the female. Still, the preponderance of the evidence seems to point to these spiders as being the same species. Of course, should you happen upon this page and recognize flaws in my reasoning, please let me know (thank you, Dr. Bradley) at once.
Among the photos Kendra sent was one of what appears to be a juvenile female. I categorize it thusly because the abdomen is not yet enlarged in the way observed in the adolescent and adult females of this species--it is, in fact, no larger than that of the adult male. The ventral abdominal markings of the female (on the right in the above photo montage) is remarkably like the sketch in Ubick et al., (2005), pg. 74, 17.71, for the genus Larinioides.
Kendra may send me additional photos, and if she does, they will be posted to provide further elucidation of the materials already provided here. She has done a masterful job already and deserves our grateful thanks.
Hopefully this material heightened your interest in the subject of arachnology. You surely noticed that much is yet unknown about North American spiders. Rather than discouraging you, that fact should make you want more than ever to get involved in helping us fill in the gaps and make strides forward in our understanding and knowledge. The field is still wide open, quite contrary to what you may have previously believed. So, as mentioned on previous pages, consider doing as Kendra did and get directly involved in pushing our knowledge forward. If you happen across a spider in North America, please photograph your discovery and send me copies of your photos. Whenever possible, every inquiry is answered immediately--I try to answer every email inquiry within minutes of reading it, and every telephone inquiry is answered on the spot, as most (alas, some emails do fall through the cracks from time to time) who call will testify.
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