Entomology 101 - Insect Physiology
(For a summary of the article, skip down to the bottom paragraph that’s under the heading “Review”)
Unlike mammals, reptiles, and fish, insects do not have bones. Instead, insects have a hard exoskeleton that functions in much the same was bones do: it protects the insect and gives it structure. The exoskeleton covers the entire outside of the insect and even covers some of the inside of the abdomen and mouth!
Insects have three body segments: the head, thorax, and abdomen. Each segment has a different function. The head mainly performs sensory input and food consumption actions. It usually contains two compound eyes, up to three simple eyes, two antennae, and a set of mouthparts. Compound eyes are each made up of tiny facets that function like pixels in an image. The larger the compound eye and the smaller the facets, the clearer and more true to life the image is. Praying mantises have large eyes with rather small facets giving them a comparatively clearer image than other arthropods. Dragonflies are another example of large compound eyes with small facets. Unlike compound eyes, simple eyes are not designed to develop an image but instead are used to detect changes in light intensity.
Unlike the eyes which are used for sight, the antennae can have several different functions. Their main function is to detect chemicals in the air, sort of like a sense of smell. The antennae are incredibly sensitive and can detect minute traces of chemicals, like pheromones, in the surrounding environment. Antennae can also be used as a tactile sensory organ in order to get a feel for an object. Some insects can even taste through their antennae!
The mouthparts of an insect are generally used to assist in the consumption of food, but can also be used in mating rituals, housekeeping, and gathering food and supplies. Additionally, mouthparts can vary wildly from order to order. Beetles have different mouthparts than flies and both have different mouthparts than bugs! Beetles usually have biting, chewing mouthparts, flies usually have spongy mouthparts, and bugs usually have piercing, sucking mouthparts. Each species has a set of mouthparts that have been adapted to that species’ specific lifestyle.
Unlike the head which has many different responsibilities, the thorax only has one: locomotion. Legs and wings are attached to the thorax on the outer portion and are connected to a network of strong muscles that span the inner portions of the thorax. Insects, by definition, have six true legs. Each leg consists of a coxa, trochanter, femur, tibia, and usually several tarsi. To make a comparison, the coxa is like our hip, the trochanter is like the ball of our femur if it had another joint in it, the femur is like our femur, the tibia is like our tibia, and the tarsi are like our feet and toes. At the end of an insect’s tarsi there is usually some form of claws that aid in moving through their environment. Just like the mouthparts, legs have many different shapes and functions to them. Raptorial legs are used to catch and hold prey like a preying mantis’s front two legs, fossorial legs are used for digging like in mole crickets, natatorial legs are used for swimming like in whirligig beetles, and saltatorial legs are used for jumping like in crickets and grasshoppers.
Insect wings are also different between the orders. Beetles actually have two sets of wings: the elytra and the flight wings. The elytra are the hard shell that opens up to reveal the flight wings when the beetle is ready to fly. Flies are the only insects with one set of wings (two wings total)! Their second set of wings have been changed into a sort of gyroscopic organ known as a haltere. Moths and butterflies have two sets of wings. Bugs have two sets of wings, but the first set is known as the hemelytra, which are similar to a beetle’s elytra but only have approximately the first half of the wings as a tougher, harder membrane. Grasshoppers have two sets of wings with their first set functioning much the same as the beetles’. In grasshoppers, their first set is known as a tegmina. Each insect has wings that are adapted to their lifestyle in much the same way that their mouthparts are.
The last piece of an insect is the abdomen. This section is primarily responsible for digestion, circulation, and reproduction. The inside of the abdomen is a conglomeration of organs and tissues. The largest part of the digestive tract resides within the abdomen. This is where most nutrient absorption occurs. Circulation is a weird phenomenon within the insect’s body as insects do not have blood like humans do. Instead, insects have a fluid known as hemolymph. Hemolymph carries nutrients, minerals, and waste around the body cavities of the insect. It does not carry oxygen. Oxygen is transported via the respiratory tract which runs through the cuticle and connects directly to the insect’s organs. Insects have a passive respiratory system which means they cannot control their breathing the same way humans can. Reproduction is largely carried out via the reproductive organs at the end of the abdomen. Reproductive organs in some insects can be wildly complex.
So, to recap, insects have three body segments, a head, thorax, and abdomen, all of which are encased in a hard outer shell called an exoskeleton. The head is responsible for sensory surveyance of the insect’s environment and for the ingestion of food and has two compound eyes, up to three simple eyes, two antennae, and a set of mouthparts. The thorax is largely responsible for locomotion and is where the insect’s six legs and up to four wings are connected. Legs, like mouthparts, come in many shapes and sizes and are adapted to an insect’s way of life. The abdomen is the site of most major digestion, circulation, and reproduction. The circulatory and respiratory systems are different in insects. Reproduction is weird and so are insects.
And there you have a crash course in insect physiology!
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-Let’s Go Find a Bug