Entomology II - Abdominal & Thoracic appendages

In Entomology I, we’ve looked at the head and all its modifications. These modifications allow insect to occupy different roles in their ecosystems - different niches and different feeding guilds. Here, we’re going to look at the appendages located on the thorax and the abdomen and look at their diversity.

THORAX The thorax is composed of 3 parts: The prothorax, the mesothorax and the metathorax. Each of the segments possesses 1 pair of leg. The wings are located on the mesothorax and metathorax.

WINGS AND FLIGHT Insects are the only invertebrates to have evolved powered flight! Though most insects have wings whether they are hidden, shortened or nonfunctional, some insects such as fleas, lice, silverfish and firebrats do not have wings. Ants may or may not have wings and same thing goes for termites. Workers and soldiers are wingless, kings and queens have 2 pairs of equally sized wings which are shed after the nuptial flight. Do you know how insect wings work❓ There are 2 types of muscles that power flight. Direct muscles as the name suggests are directly connected to the wings to power flight. Dragonflies and damselflies of the order Odonata, have direct flight muscles. This allows them to use their wings independently, out of sync, enabling rotations on spot and flying on the same spot. This is what makes their flight so mesmerizing. Note that their wings are also very reticulated, they have a lot of veins running across them, which makes them more rigid as well.

Indirect flight muscles on the other hand are not directly connected to the wings, but rather to the thoracic box. Muscle action deforms the thoracic box, which in turn moves the wings. In other words, they can't be used out of synch.

Do you know how scientists study flight in animals❓ They use a method called digital particle image velocimetry, or DPIV for short, to precisely analyze the flow field around the wing. DPIV uses a high-resolution imaging to track the movement of marker particles, moving in the air as the insect flies. This data shows the flow field and can be combined with measurements from an attached strain gauge, which measures the forces acting on the insect. To sum up, DPIV can both inform on the flow field and force acting on the insect during flight. I see it as a pretty interesting scientific tool that could be used to make better drones and other flying robots, based on insect's flight!

DPIV of a small bird in flight

LEG MODIFICATIONS We’ve looked at the wings, but our exploration of thoracic appendages is not over yet as we need to explore the variety of legs. Legs are also found on the thorax, 1 pair per segment as we saw. Insect’s legs can be used for jumping – saltatorial with enlarged femur, for digging – fossorial with enlarged tarsi, for swimming – natatorial (in which case there are hairs – hydrofuge on the tibia and tarsi), and finally, for hunting - raptorial with spines on the femur and tibia. The basic leg is used for running – cursorial with long femur, tibia and tarsi.

From left to right, top to bottom we have: Grasshopper with enlarged femur Dung beetle with enlarged tarsi Preying mantis with raptorial front legs Diving beetle with hydrofuge

ABDOMEN Finally, let’s take a look at the abdomen. At the extremity of the abdomen, you’ll find the male’s aedeagus (phallus), the female’s genital pore - (gonopore) and the cerci, which can also be modified like most things we’ve seen so far. Cerci are often used for chemical and mechanical perception. In earwigs, the male grasps the female with its cerci in order to insert its aedeagus into her gonopore for copulation. These cerci form pincers also called forceps and can be further used for defense, prey handling, courtship display and thanks to the telescopic proprieties of their abdomen, they can use their forceps in all direction to help in the folding of the large hindwings. The cerci can also be modified into an ovipositor in females, in which case it’s used to burry eggs on the ground, inject them into a plant or a host, glue them to any substance including the body of a mate, drop them like bombs in the water or on top of a host.

From top to bottom and left to right we have: Ichneumon wasp with an extremely long ovipositor for egg laying into a host. Katydid ovipositor Mayfly cerci Earwig forceps

SOURCES - My University lectures - Lectures from the University of Alberta: Bugs 101: Insect-Human Interaction. Can be found on Coursera - 2 Books on entomology which names I don't remember. They're in my uni library, I'll add the titles when this whole covid mess is over and I can go back to uni.

- Lifeyard