Lights, Camera, Spider

Giant House Spider -  (Eratigena atrica). The presence of palpal bulbs indicates that this is a male. These are used to transfer sperm to the female seminal receptacles during mating. Most of the  house Spiders that are found in houses in autumn are males searching for females.

Father Ted: "OK, one last time. These are small  (holds up toy cows)... but the ones out there (points out of the window) are far away. Small... far away... (Dougal shakes head in confusion) ah forget it!” See the excerpt here I can imagine Father Ted would have an even harder time trying to explain these spider close ups. Father Ted: "This is small  (points to spider)... but this one (points to the photo) is highly magnified. Small... magnified... (Dougal shakes head in confusion) ah forget it!”

My makeshift spider 'studio'

Garden Spider (Araneus diadematus). The protruding structure at the base of the abdomen is the epigyne. This is used to receive  and direct the palpal organ of the male during copulation. The form of the epigyne varies from species to species and corresponds to male's palp. There are lots of examples of this in nature - similar species have genitalia which only 'fit' members of the same species.

In this way intra-species mating is prevented. There are many example of this in moths. Pairs of species often look identical to the naked eye, and can only be separated by microscopic examination of the genitalia.

How has this evolved? The lock and key hypothesis proposes that genitalia evolve by hybridization avoidance. Hybrids being inferior from a survival point of view, therefore less likely to pass on their genes. This drives the evolution of male genitalia with a proper mechanical fit to female genitalia.

Alernatively, the sexual selection hypothesis, proposes that divergent evolution of genitalia is the result of sexual selection driven by varying paternity success among males.

Get this