Mosquitos Part III: Larvae

Eggs can be enourmously hard to find, not “just ’till you get the hang of it” hard but, seemingly “this is a snipe-hunt right?” hard. The search for eggs might be quite enough to put someone off their search. Among the easiest eggs to find are those of the genus Anopholese. These small dark eggs float together in groups parallel with the water surface and are not infrequently (and justly) described as asemblies of asterisks. The dilligent might also find the eggs of Culex species which take the form of floating rafts where the individual cigar-shapped eggs are packed closely together forming rafts. They’re arainged vertically in groups of several dozen or more.

The larvae are much simpler to find. We’ve seen before where to look, so now a few pointers on sucess in hunting. Larvae of most mosquitoes will respond the same way to the threat of any preaditor. If the water in which they repose is disturbed, even if a shadow falls over it the larvae will flee to the bottom of their habitat. This behaviour is not such a hinderence when hunting larvae in a shallow retreat, any handy jar will serve for collecting. In deeper habitates it may help to use an inexpensive rubber bell baister. Gathering larvae at the surface is a simple matter of using an empty vessel to create a vacuum. If one only manages to draw in a few larvae at a time the bottom seeking behaviour makes condensing them a simple matter of pouring out some of the extra water while the larvae cluster at the bottom. When a fair supply has been gatherer tightly close the vessels and travel immediately back to home base. Make sure to leave about one quater of any vessel empty as airspace so that the air-breathing microbes the larvae eat do not suffocate, the larvae themselves will breath at the surface.

Once home transfer the larvae into a breeder and place it somewhere where it is convienient to observe the water surface from the side. The larvae can be identified to a genus level rapidly with the naked eye. At rest the larvae will gather at the surface of the water to breath, this is the chief aid in identification. Anopholese larvae will lay parallel with the sruface while both Aedes and Culicinae will seemingly hang from the surface at a 70 degree angle. The large dark head hangs towards the bottom filter feeding while the posterior breathing tube projects towards the surface.

Under a handlens one may note that some larvae have an obviously longer breathing tube. These same larvae will also appear to have longer, darker, and more obvious hairs protrudeing from their body segments. Identification based on the angle assumed with respect to the surface may only be made on what are called fourth instar larvae.

After hatching the larvae will be small, just a millimeter or so. They will grow rapidly through four stages of immaturity, called instars. The largest and final instar is what most larval identification standards are based upon. In the photo below can be seen some third and fourth instars of the Culex genus. The shalow water in the petri dish doesn’t allow them to orient normally to the water surface but it’s clear from the abundance and lengths of the hairs on the body segments and head particularly that they’re Culicinae.

At the bottom of the image one can see a visibly different larvae. The dark dorsal projection from the tail end (the breathing tube) is quite a bit longer and despite being close to the edge of the dish the visible shadow makes it clear that the long hairs common to the other larvae are fewer and less significant. This indicates the likely presence of an Aedes larvae, positive identification this is not, but after it matures fully identification will be obvious.

One thought on “Mosquitos Part III: Larvae

  1. Pingback: Mosquitos Part IV: Techniques | vade mecum microscope

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