Mosquitoes Suck!

No, really. Mosquitoes suck. Some may think when a mosquito sticks its mouthparts into the skin the blood would just start flowing right in, but that simply is not the case. If you were to stick a straw into a river, the water would not instantly move up into the straw. This is because of inertia: the water is moving in a particular direction, and to get it to move in a different direction a force must be applied. An additional challenge for the mosquito is the need to break the surface tension caused by the adhesive/cohesive properties of water which makes up much of the blood it ingests. So, within the head of the mosquito there is a double pump system which allows them to suck.

Just inside the head, the feeding tube is anchored on either side, while a muscle group attached to its upper surface contracts and expands the volume of the tube forming the smaller cibarial pump. The increased volume decreases the pressure and initiates blood flow. As the blood moves through to the next section of the feeding tube, a valve is in place just beyond the cibarial pump which prevents the backflow of the blood. Toward the back of the head, a set of three muscle groups expand the volume of the tube forming the larger pharyngeal pump.

Fig. 1 – Diagram (from Snodgrass, 1959) indicating the locations of the cibarial pump (CbP) with its associated muscles (5), and the pharyngeal pump (PhP) with its associated muscle (8,11).

To create the flow of fluid into the head, the muscles on the pharyngeal pump contract while those on the cibarial pump relax. Then as the pharyngeal pump relaxes, the muscles of the cibarial pump contract. These alternating contractions along with the valve mechanism produce a positive flow of blood into the mosquito’s digestive system.

Fig 2. – Imaging of mosquito pump system (Kim, 2012)
C: 3-D reconstructed image showing cibarial pump (CP), pharyngeal pump (PP) and the anterior pharyngeal valve (Vap)
D: synchroton X-ray image of mosquito head (lateral view)

The muscles which pull on the feeding tube for both the cibarial pump and the pharyngeal pump are connected to the inner surface of the head capsule. To provide structural support, a pair of tentorial arms within the head keep it from collapsing under this strain. The tentorial arms also resist the external forces put on the head by the actions of the mouthparts.

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

It has recently been discovered through x-ray imagery that the mosquito can utilized two different modes of action with this pump system depending on the type of feeding involved (Kikuchi, 2018). When drinking water or nectar, the mosquito will use continuous, small reciprocal strokes between the two pumps. When taking a blood meal, however, the mosquito will employ periodic, large burst strokes from the pharyngeal pump which significantly increases the rate of flow. This ‘burst mode’ of pumping requires a massive increase in power, but significantly reduces the time necessary to take a sufficient blood meal – an obvious advantage when robbing from something which can kill you.

While I don’t expect this information will cause anyone to like mosquitoes any better, one should at least appreciate the ingenuity of this system, and marvel that it all works and fits inside the head of a mosquito which is about one millimeter in diameter. Although the cibarial and pharyngeal pump systems are not uncommon among insects, the system found in mosquitoes is uniquely modified to enable them to utilize blood as a food source. This adds to the complexity discussed in an earlier post regarding the engineering found within mosquito mouthparts. Without this uniquely modified pump system, the specialized mouthparts would be of little use, but such modifications to the pump system would not be in place without the highly engineered mouthparts. So, the presence of this pump system further resists a Darwinian account of the feeding mechanisms of mosquitoes.

References

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).

Kikuchi, K., Stremler, M.A., Chatterjee, S. et al. Burst mode pumping: A new mechanism of drinking in mosquitoes. Sci Rep 8, 4885 (2018) doi:10.1038/s41598-018-22866-w

Kim BH, Seo ES, Lim JH, Lee SJ. Synchrotron X-ray microscopic computed tomography of the pump system of a female mosquito. Microsc Res Tech. 2012 Aug;75(8):1051-8. doi: 10.1002/jemt.22030. Epub 2012 Mar 14.

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

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