Mosquitos Bite!

“I hate mosquitoes! What are they good for anyway?” This is the typical response I get when people learn I did my Masters research on mosquitoes. While mosquitoes probably deserve most of the reprehension they receive from humans, one thing which can be said on their behalf is they are an example of masterful engineering. So, set aside your mosquito animosity for a few moments, and appreciate with me some of the intricacy and design to be seen in this little monster.

Anyone who has taken a few moments to observe a mosquito as it prepares to take a blood meal from their arm has noticed they have a long needle-like structure emanating from their head. This is known as its proboscis. What most don’t realize is that the entire structure is not inserted under the skin. What you are looking at is a sheath which encloses and protects the stylets which make up the feeding tube that is inserted under the skin. This sheath known as the labium (Fig. 2, E and G, Lb) is normally a flattened “lower lip” in most insects, but here has been curved around into a tube to enclose the stylets. This labium must be folded out of the way when the mosquito takes its blood meal (Fig. 1).

Fig 1. – Successive stages of penetration of stylets of a female mosquito feeding on blood (Snodgrass, 1959)

The feeding tube which is inserted under the skin is very narrow – less than the diameter of a human hair. This tube is also not a singular structure, but instead is composed of six stylets – each playing a unique role in the process. To first get through the skin, two of the stylets referred to as maxillae possess sharp serrated edges which cut through the different layers of tissue (Fig. 2, H, Mx).

Fig. 2 – Mouthparts of an adult mosquito (Snodgrass, 1959)
Hphy, hypopharynx; Lb, labium; Lm, labrum; Md, Mds, mandible; Mx, Mxae, maxillae; Prb, proboscis.

When the mosquito inserts its feeding tube, it can’t just stick it anywhere. As seen in Fig. 1, the stylets must be inserted into a blood vessel. To find the blood vessel, the tip of one of the stylets known as the labrum (Fig. 2, Lm) is equipped with sense receptors (Fig. 3, Lbl) which detects a chemical found in the blood recently identified as 4EP (4-ethylphenol), a prostaglandin (Choo, 2015).  The labrum is equipped with a sharp tip which is also effective in cutting through tissue (Fig. 3, Lig). A channel is formed by the labrum through which the blood will flow (Fig. 3, LG) once inserted into a blood vessel.

Fig. 3 – tip of the labium (Snodgrass, 1959)
LG, labial gutter; Lbl, labellum; Lig, ligula

Another pair of stylets which aid in the process are referred to as mandibles. In chewing insects, these form grinding surfaces to break up the insect’s food before it enters the gut. In the mosquito they have been refashioned into slender paddle-like stylets which have the dual function of moving tissue out of the way as the feeding tube is inserted under the skin, and enclosing the u-shaped groove of the labrum.

Damaging a blood vessel causes an immediate physiological response to prevent blood loss. Utilizing chemicals within both the plasma and platelets, a biochemical cascade erupts resulting in a patchwork of fibers and blood cells we call a blood clot. This biochemical reaction would normally put the mosquito out of business, except the mosquito is prepared for such things. Another of the stylets known as the hypopharynx (Fig. 2, H, Hphy) possesses a small groove which is a canal for saliva the mosquito releases into the blood vessel. The saliva contains chemicals which act as an anticoagulant and a vasodilator. The presence of these foreign proteins is what results in the welt you sometimes experience after the mosquito has parted ways.

As you might imagine, some amount of force is needed to press this cluster of stylets into the skin. This probing force is not only exerted on the skin, but on the mosquito’s head as well. The mosquito’s head which is less than 1 mm in diameter would risk collapse from such efforts if it were not for internal bracing structures known as tentorial arms (Fig 4, tnt). In addition to maintaining structural integrity, the tentorial arms act as anchor points for many of the muscles involved in moving the mouth parts.

Fig 3. – Internal imaging of mosquito head (Anopheles sinensis) using synchrotron X-ray microscopic computed tomography (Ha, 2015)
Tnt, tentorial arm

While you may not enjoy the overall result of this intricate assemblage of parts, it should be recognized that the complexity present in this system resists explanation by the evolutionary processes of random mutations and natural selection. There are a host of genes which must be altered to accomplish the modifications described above. In order for any one of the mouth part modifications to improve survival and be preserved by natural selection, all the other modifications would need to be in place as well. The mosquito would fail to obtain its blood meal if any one of these structures was missing or misshapen. Such interdependent arrangement of parts in a structure has been referred to as irreducible complexity.

In addition to the coordinated mutations required to yield the mouthparts, there are mutations in other systems necessary to support the feeding mechanism. The mouthparts are modifications of the exoskeleton, but as noted above, there are modifications to the digestive system and nervous system as well. The salivary glands of the mosquito are producing very unique proteins (an anti-coagulant and vasodilator) atypical to the feeding requirements of other insects. The nerve receptors on the mosquito’s labrum are able to detect blood proteins. The lack of the chemical contributions by these systems would render the mosquito’s mouth parts useless

It would seem to be a mighty tall order to explain the mosquito’s feeding mechanism to a multitude of coincidental gene mutations. By our human experience, when we observe irreducibly complex structures and interdependent systems, we know that such things exist as a result of a tremendous amount of foresight and planning. Evolutionary processes operate without any ability to predict future needs and produce novel structures to meet those needs. A more plausible explanation is that the mosquito’s mouthparts exist as a result of intelligent agency.

Some may question how intelligent the designer of this complex system was, but that is more a subject for theologians and philosophers. What is inescapable is an appreciation for the engineering which is accomplished here. So, the next time a mosquito lands on your arm, by all means, swat it. But hopefully, you will remember all the intricate structures you just squashed!

Works cited

Choo, Young-Moo et al. “Multitasking roles of mosquito labrum in oviposition and blood feeding.” Frontiers in physiology vol. 6 306. 29 Oct. 2015, doi:10.3389/fphys.2015.00306

Ha, Y.-R. et al. Comparison of the functional features of the pump organs of
Anopheles sinensis and Aedes togoi. Sci. Rep. 5, 15148; doi: 10.1038/srep15148 (2015).

Snodgrass, R. E., The Anatomical Life of the Mosquito, Smithsonian Miscellaneous Collections, Vol. 139:8 (1959).

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