Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

On account of the importance of the group in this country and the desirability of the student being able to determine material in various stages, we show in the accompanying figures the characters most used in classification.

The larvæ ([fig. 73]) are elongate, with the head and thorax sharply distinct. The larval antennæ are prominent, consisting of a single cylindrical and sometimes curved segment. The outer third is often narrower and bears at its base a fan-shaped tuft of hairs, the arrangement and abundance of which is of systematic importance. About the mouth are the so-called rotary mouth brushes, dense masses of long hairs borne by the labrum and having the function of sweeping food into the mouth. The form and arrangement of thoracic, abdominal, and anal tufts of hair vary in different species and present characteristics of value. On either side of the eighth abdominal segment is a patch of scales varying greatly in arrangement and number and of much value in separating species. Respiration is by means of tracheæ which open at the apex of the so-called anal siphon, when it is present. In addition, there are also one or two pairs of tracheal gills which vary much in appearance in different species. On the ventral side of the anal siphon is a double row of flattened, toothed spines whose number and shape are likewise of some value in separating species. They constitute the comb or pecten.

The pupa ([fig. 139, b]) unlike that of most insects, is active, though it takes no food. The head and thorax are not distinctly separated, but the slender flexible abdomen in sharply marked off. The antennæ, mouth-parts, legs, and wings of the future adult are now external, but enclosed in chitinous cases. On the upper surface, near the base of the wings are two trumpets, or breathing tubes, for the pupal spiracles are towards the anterior end instead of at the caudal end, as in the larva. At the tip of the abdomen is a pair of large chitinous swimming paddles.

As illustrative of the life cycle of a mosquito we shall discuss the development of a common house mosquito, Culex pipiens, often referred to in the Northern United States as the rain barrel mosquito. Its life cycle is often given as typical for the entire group, but, as we have already emphasized, no one species can serve this purpose.

The adults of Culex pipiens hibernate throughout the winter in cellars, buildings, hollow trees, or similar dark shelters. Early in the spring they emerge and deposit their eggs in a raft-like mass. The number of eggs in a single mass is in the neighborhood of two hundred, recorded counts varying considerably. A single female may deposit several masses during her life time. The duration of the egg stage is dependent upon temperature. In the warm summer time the larvæ may emerge within a day. The larvæ undergo four molts and under optimum conditions may transform into pupæ in about a week's time. Under the same favorable conditions, the pupal stage may be completed in a day's time. The total life cycle of Culex pipiens, under optimum conditions, may thus be completed in a week to ten days. This period may be considerably extended under less favorable conditions of temperature and food supply.

Culex pipiens breeds continuously throughout the summer, developing in rain barrels, horse troughs, tin cans, or indeed in any standing water about houses, which lasts for a week or more. The catch basins of sewers furnish an abundant supply of the pests under some conditions. Such places, the tin gutters on residences, and all possible breeding places must be considered in attempts to exterminate this species.

Other species of mosquitoes may exhibit radical departures from Culex pipiens in life-history and habits. To control them it is essential that the biological details be thoroughly worked out for, as Howard, Dyar, and Knab have emphasized, "much useless labor and expense can be avoided by an accurate knowledge of the habits of the species." For a critical discussion of the known facts the reader is referred to their monograph. We shall confine ourselves to a few illustrations.

The majority of mosquitoes in temperate climates hibernate in the egg stage, hatching in the spring or even mild winter days in water from melting snow. It is such single-brooded species which appear in astounding numbers in the far North. Similarly, in dry regions the eggs may stand thorough dessication, and yet hatch out with great promptness when submerged by the rains. "Another provision to insure the species against destruction in such a case, exists in the fact * * * that not all the eggs hatch, a part of them lying over until again submerged by subsequent rains." In temperate North America, a few species pass the winter in the larval state. An interesting illustration of this is afforded by Wyeomia smithii, whose larvæ live in pitcher plants and are to be found on the coldest winter days imbedded in the solid ice. Late in the spring, the adults emerge and produce several broods during the summer.