Entomology for medical officers / by A. Alcock.

  • Alfred William Alcock
Date:
1920
Catalogue details

Licence: In copyright

Credit: Entomology for medical officers / by A. Alcock. Source: Wellcome Collection.

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    those species that breed in pitcher-plants and things of that kind—it is plentifully covered with tufts of hairs, but usually hairs are conspicuous only at the sides of the segments and on the sternum of the last segment, where they form a sort of fin. On the dorsum of the 8th segment the main tracheal trunks open, either in a hollow protected by adjustable folds and plates (Fig. 16), as in Anopheles, or in a distinct tube as in all other Culicince. This breathing-tube has its orifice loosely fitted with valves, and on its posterior surface there is usually a double row of spines. On the sides of the 8th segment there are generally some scales, or spines, the number and form of which are specific. The 9th segment is small, and generally ends in four tracheal gills of varying size, which surround the anus. The principal features of the internal structure are shown in Fig. 15. The broad digestive-tube passes straight through the body; in the thorax it expands in glandular caeca, and in the 6th segment it receives the five Malpighian vessels; the two main tracheal trunks run dorsal of and parallel with it, from the spiracles of the 8th abdominal segment to a point in the thorax where the breathing-openings of the pupa will subsequently appear, at which point they suddenly become contracted. The locomotion of the larva is effected by energetic wriggling. In repose the larva of Anopheles (and of a few other species) lies horizontally, usually at the surface of the water; but in most other forms the resting larva hangs head downwards with the tip of the breathing-tube at the surface. The larva moults three times, becoming full grown in the most favourable conditions of food and warmth in one to three weeks, but cold and other unfavourable circumstances may cause the larval stage to be prolonged to several weeks, and in northern latitudes some species of Culex and Anopheles may remain in the larval stage throughout the winter. When the larva is full grown many of the adult structures— particularly the legs—can be made out below the skin. The full-grown larva casts its skin a fourth time and becomes a pupa (Fig. 19), in which stage it resembles a miniature lobster deprived of appendages. The head and thorax form a single mass, in which the eyes, antennae, legs,
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