Spider identification

Distinguishing males, females, and immatures

For most species of spiders, there is virtually no morphological difference between the sexes until maturity. Most spiderlings emerge from the egg sac looking very much like miniature models of the adults. Because they have an exoskeleton, as spiderlings grow, they must molt in order to advance to the next larger stage of development, leaving behind a shed skin similar to snakes or insects like grasshoppers and earwigs. Except in the few cases where there is great disparity in the size of mature males and females, it is not possible to distinguish the sex of a spider during most of the immature stages. As spiders reach their final or near-final molts, sex determination becomes apparent.

The key feature for sex determination in spiders is the pedipalps, commonly referred to as the "feelers." Pedipalps are jointed leg-like structures anterior to the first pair of legs. They are much shorter and thinner than the legs, and they have 6 segments whereas spider legs have 7 segments. Pedipalps are covered with fine hairs that function as sensory organs. The pedipalps are undifferentiated in females and most immatures, but in mature males, they develop into a complicated structure that is used to deliver sperm to the female. Typically, sex determination before the molt to maturity (the terminal molt in all but a few rare species) is not feasible. The male mating organ usually becomes apparent in the penultimate molt of the male, but at this stage, the palp lacks any surface features, so it is not yet useful for taxonomic work. In mature males, the surface structures that aid in directing the mating organ into the female genital opening are present and are useful for species determination.

For most female spiders, maturity is indicated by a hardened patch of sclerotized tissue on the anterior ventral portion of the abdomen called the epigynum. For a few primitive species (e.g., Kukulcania geophila), there is little external indication of maturity in females, and dissections are usually necessary for species determination. However, the few such species included in this tool can be identified by other more noticeable features, so dissection should not be necessary.

It is critical to be able to differentiate males, females, and immatures in order to accurately identify spiders. Most spider keys base features on mature specimens, especially in cases where immatures undergo many coloration changes from egg sac to maturity. While it may be possible to identify immatures, there may be slight changes in structures or coloration as a spider matures. If two or more species in the same genus have similar body shape, proportional leg dimensions, and coloration, then they may not be distinguishable as immatures.