Mosquito Anatomy Inside and out

Tomes have been written about mosquito morphology and taxonomy. Because of their economic importance, there is probably more known about the mosquito than any insect other than the giant silkworm, the honeybee and, thanks to E.O Wilson, ants.

Mosquitoes are insects.  Generally speaking, insects have a hard shell, three body parts, six articulated legs, two antennae and four wings.  Their mouthparts are pairs of modified legs.

The outer shell of the mosquito is called an exoskeleton.  It is made up of solid plates of a hard protein called chitin. There is a waxy coating to prevent dehydration. Removing the coating or damaging it with abrasives such as diatomaceous earth will cause the mosquito to lose body fluid through the exoskeleton.  On the inside of the shell are bumps and ridges for muscle attachment.  In order to move, there is another protein which forms stretchy sheets between the hard plates.  This material is called elastin.

The three body parts of the mosquito are the head, the thorax and the abdomen.  The head carries the antennae, the two compound eyes and the occelli, or simple eyes. The compound eyes are actually many eyes in one location.  If a person took a handful of drinking straws and looked through them, it would approximate the information coming to the mosquito brain from the compound eyes. This structure, although not so good for forming one single image, is excellent for motion detection. The ocelli are used for light detection. It is more complicated than simple light and dark; the angle and amount of light are used to orient the mosquito in relation to the sun.

The mosquito antennae have hairs with specialized functions. Some are used to detect wind speed and some are actually chemoreceptors which act like many tiny noses. Mosquito “noses” detect carbon dioxide which is exhaled by their potential meal. Some mosquitoes are very choosy about which animals they dine on. They are able to determine the suitability of the animal by the amount of carbon dioxide and the combination of other chemicals given off.

The mosquito mouth is actually pairs of modified legs. In the female mosquito, the pieces fit together to form a tube. The outer pair actually acts as a drill. As the mosquito searches for a capillary bed to dine from, the sides of the mouth alternate punching down through the skin and fatty tissue into the blood pool. The outer pair of mouthparts also contains chemoreceptor pits, which are the mosquitoes last line of defense if she is about to take a blood meal from an unsuitable host. The tongue is a tube which conveys the meal into the foregut. Mosquito saliva contains an anticoagulant. Many of the disease organisms carried by mosquitoes live in the salivary glands and are injected into their host with the mosquito saliva. Male mosquitoes do not draw blood. They live exclusively on nectar and their mouthparts are not adapted for blood meals.

The thorax of the mosquito carries six legs. The legs are jointed, as are the legs of all arthropods. Mosquitoes actually have the ability to taste and to detect temperature with their “feet.” They have chemoreceptors on their tarsi which detect the chemicals in the skin and on the hair of the animal they are walking on. There are also some areas on the exoskeleton which are extremely temperature sensitive. The mosquito uses this temperature detection to locate a place where the animal’s blood is closest to the surface of the skin. Areas with a good blood supply are warmer and the mosquito can locate the best spot to feed.

The thorax also carries the wings. Mosquitoes, as do most of the other flies in the order Diptera, appear to only have two wings. On closer inspection, the second pair of wings has been modified over time into specialized bristle. This bristle is called a haltere. It appears to be a ball on a stalk. The mosquito uses this structure for balance, positioning and flight stability.

The final part of the mosquito’s body is the abdomen. The abdomen of a mosquito has larger sheets of elastin between the chitinous plates because the abdomen is designed to expand with a blood meal or when fully engorged with eggs. The other interesting feature of the abdomen is the breathing apparatus. Much like the 1949 Roadmaster, mosquitoes have holes in their sides. These portholes are called spiracles, and they have a plate which can be closed in the presence of any substance which would be harmful to the insect. Insects do not have lungs. In their respiratory scheme, the spiracles are connected directly to the major muscle bundles within the insect’s body by tubes called tracheae.

Inside the mosquito is a fascinating place. An insect is a chamber filled with liquid which sloshes generally from the head to the raster (the last bottom plate at the end of the abdomen). The blood is returned to the head via a heart which is actually a long, pulsating tube.

In addition to the muscles, the tracheae and the salivary glands, which were described earlier, there are some other critical structures. In the head is a six-lobed brain. Two lobes are dedicated to vision, two are dedicated to locomotion and the remaining two enervate the remaining functions of the body.

The digestive system winds its way through the mosquito. It is divided into the foregut, where the blood meal is bundled as a discrete unit in a permeable membrane. In the midgut, digestive enzymes penetrate the meal bundle and begin extracting nutrients. In the hindgut the nutrients are moved out from the digestive tract into the blood which will carry them to the tissues of the insect. Waste materials move into the hindgut and are excreted.

The reproductive tract of mosquitoes has the same form and function as the reproductive tract of almost every other animal on earth. Mosquitoes are not one of the insects which have live birth, so the end product is a raft of eggs. One feature of egg-laying creatures which does not occur in mammals is a spermatheca. This structure is essentially a bag of sperm. During mating, sperm is deposited in the spermatheca. If there are subsequent matings, the sperm does not mix, instead it is in layers. Sperm from the first mating will be used until it is gone.

For most readers, this piece should provide a significant overview of one of the most economically and medically important insects on our planet. However, if this quick tour of mosquito anatomy, inside and out, has inspired you to learn more, one excellent technical reference is “The Insects: Structure and Function,” written by R. F. Chapman.