A female mesh web spider feeds on a gnat.

Pretty typical spider and fly scene, right? Now, what if I told you I could add one more spider and still have one spider? (Nope, no intraguild predation!) Let’s look at the spider behind the blog post curtain…

These spiders look very different, but they are the same species. The red one’s big pedipalps, or simply palps — the black appendages at the front end of the spider — give it away as the male. These structures have developed as a container of sorts to transfer sperm to the female spider.

Voila! One spider for the price of two! Or just one male and one female of the same species. This pair illustrates a stark case of sexual dimorphism. Wikipedia tells us:

Sexual dimorphism is a phenotypic difference between males and females of the same species, meaning that there are obvious differences between the male and female of the species. Examples include differences in morphology, size, ornamentation and behavior.

These types of differences can make identification of arthropods, especially for your non-experts, a hazardous exercise. Given that the spiders were coexisting peaceably in such tight quarters, plus that I have on other occasions photographed these same spiders mating, I feel pretty confident calling them as one species. It does help to catch sexually dimorphic bugs the act. For the novice, for instance, finding a lonely average-looking, banally brown male moth might make it hard to place with its wingless female counterpart.

Another thing we can gather about these photos, is that, even in close quarters, it is difficult to get sexually dimorphic spiders into agreement. I mean focus. </snark>

The female has a puffier abdomen and much less pronounced pedipalps.