Sexual Dimorphism in Arachnids

          Dimorphism: pron. (primarystress)dimacrprimarystressmodotrsecondarystressfizschwam. Etymology: International Scientific Vocabulary dimorph- (from Greek dimorphos) + -ism
: the condition or property of being dimorphic or dimorphous : as a : difference (as of form, color, size) between two individuals or kinds of individuals that might be expected to be similar or identical <the floating and submerged leaves of aquatic plants may exhibit considerable dimorphism> <in certain marine invertebrates sexual dimorphism is so extreme that the male is reduced to a minute parasite in the kidney of the female> -- compare POLYMORPHISM b : crystallization in two different forms "dimorphism." Webster's Third New International Dictionary, Unabridged. Merriam-Webster, 2002.

Sexual dimorphism in arachnids deals with significant differences found in the sexual forms of various species, particularly in those anatomical features which intuition suggests should be identical. In our focus, in the arachnids, is on spiders. Within that biological grouping we find a number of very interesting sexual dimorphisms. Some of these are observed only in isolated genera or species, while others are common to all genera and species within the entire Araneae.

For example, sexual dimorphism is demonstrated in the morphology of chelicerae (the jaws and fangs) and palpi (the spider's diminutive, anterior, leg-like appendages), the disparity in the number and variety of chemoreceptive palpal sensilla, and in the overall body sizes of the female vs. the male. Of course, other differences also exist between the sexual forms, such as in the structures of the external genitalia, and the coloration of the dorsum, but because such differences are--for the most part--entirely expected, they do not rise to the status of sexual dimorphism.

Cheliceral variations between the sexual forms of certain spiders may be dramatic. The chelicerae of the male magnolia green jumper (Lyssomanes viridis), for example, project outward horizontally as far as the carapace is long, yet the female's chelicerae are, by comparison, unremarkable. Similarly, the male common zebra spider (Salticus scenicus) sports chelicerae five times larger than the female's. However, size is not the only distinction for S. scenicus, as the male's oversized chelicerae also lack a venom duct and, being incapable of envenomation, cannot be employed in the traditional sense but are used instead as skewers.

Spiders are not unique in their sexual dimorphisms, but sometimes parallel--more or less--the anatomical disparities found in insects. For example, male hexapod antennae are provided with more sensilla than those of the female. In the spider, the analog of the insect antenna is the palpus, and male palps are known to have close to three times as many chemoreceptive sensilla as those of the female. The way the male uses the palpi to track females of like species is a crucial factor in the development of this attribute, as a successful rendezvous is not likely unless the male can detect miniscule traces of the female's species-unique pheromones in silk the male finds while conducting a random search for a mate.

The possession of extra-sensitive palpi is eclipsed by the unusual way the male palp terminates. In both the haplogyne and entelegyne female, the palpi appear and function as diminutive legs, having all the segments of an ordinary leg save one (the metatarsus is absent). In the typical haplogyne male, the external appearance of the palp is often identical to that of the female, as relatively small haplogyne embolus is not much larger than an ordinary tarsus and the palpal segments are otherwise normal in shape and size. In other haplogyne males, e.g., the southern house spider (Kukulcania hibernalis), one or more of the palpal segments are exaggerated in length. In the entelegyne male, however, the palpal embolus is so much larger than the other palpal segments that it appears swollen.

The embolus, in any case, serves as the instrument by which the male inseminates the female. When, in 1843, the German physicist and naturalist Franz Anton Menge discovered this functionality, he assumed that internal ducts conveyed sperm from abdominal vesicles to the palps. Later observations showed, instead, that the male mechanically charged each palp with sperm by pressing it into a specially constructed sperm web, on which the spider had previously deposited a drop of sperm exuded from its epigastric furrow. This process of sperm induction, and the subsequent application of the palps to the female's epigynum, in a complicated and--for the male--dangerous procedure whereby the sperm becomes safely lodged in the female's internal seminal receptacles, is one of the most intriguing behaviors observed in the arthropods.

Finally, in many species of spiders the male is dwarfed in size by the female.  Not always is this true, and in fact among the Lycosidae--the wolf spiders--the male is often much larger than the female. However, in the Theridiidae--the cobweb weavers--and the Araneidae--the orbweavers--the female is typically 2-50 times the size of the male, dimensionally speaking, and as much as 1000 times heavier. When the sexual forms of these spiders are found in the same web, they may appear to be entirely different species. Because the female is so much larger and impressive, it is the female whose photograph gets taken while the male is largely (!), if not entirely, ignored.


Did this brief definition assist you in your study of arachnology? I hope it did. Ignorance about our arachnid friends--they are, in the main, rather beneficial, not the enemies most of us imagine them to be--is rife. 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. Step forward, walk in Rachel's footsteps, and do something positive that cranks our knowledge upward. 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 make the effort to commune with me will testify.