Behind the insects head, we encounter the next insect body region, the thorax. The thorax consists of three segments. The segment closest to the head is the prothorax, followed by the mesothorax, and finally the metathorax which is adjacent to the abdomen. Note that each thoracic segment bears a pair of legs, wings, if present in adults are only found on the meso and the metathorax. The final and most posterior body region of an insect is the abdomen, which contains most of the internal organs. The number of segments that make up the insect abdomen varies by species. Primitively, the insect abdomen has 11 segments while more derived groups typically have fewer. External genitalia use in mating are often found towards the posterior end of the abdomen. The genitalia of male and female insects have enormous structural variation and each example is often unique to a particular species. The male sexual organ is called the aedeagus, and is usually protected inside the body until mating begins. A major part of the aedeagus is the penis, a tube-like organ which delivers sperm. The Aedeagus may also include small claspers which allow the male to hold onto the female during copulation. Some female insects may have a visible ovipositor, a structure used to deposit eggs. Other females have telescoping ovipositors that can retract back into the body between egg-laying cervix. Ovipositors can have sensory structures that allow females to detect chemical, mechanical or thermal cues to recognize optimal egg-laying sites. In some parasitic Hymenoptera such as the ichneumonid wasps, the ovipositor can be extremely long, especially if eggs need to be deposited in an insect hosts that is hidden deep within plant tissues. Some females use their ovipositor to bore through solid wood to reach a host. In many other Hymenoptera species, the ovipositor is modified into a stinger. If you've ever been stung by a bee or wasp, you are already familiar with it. In addition to external genitalia, some insects have sensory appendages at the end of their abdomens called cerci. [inaudible] on the surface of some cerci can detect air currents created by predators approaching from behind. They can also be modified to form pincers as seen in earwigs, or for reproductive purposes like the claspers in dragonflies respectively. In many insects, the cerci are greatly reduced or lost completely. Now that we have explored the external morphology of insects from the head to the tip of the abdomen, in the next video, we will begin to look at what lies underneath the exoskeleton.