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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    tubules—of varying number, which usually open into the intestine just behind the stomach. The Respiratory System.—Breathing is carried on by a system of air-tubes, or trachece, which, except in some larvae, open at regular intervals on the sides of the thorax and abdomen. The openings are known as spiracles, and there are commonly ten pairs of them (but often less), namely, two larger pairs on the thorax, and eight smaller pairs on the anterior abdominal segments. The spiracles may be opened and closed by muscles and valves. From the spiracles short wide tracheae pass inwards and, after branching, break up into copious minute ramifications which carry air to every part of the body. The main tracheal branches almost always unite to form a pair of tracheal mains that run longitudinally on either side of the body, communicating with one another by transverse branches, and sometimes expanding to form air-sacks. The larger tracheal tubes are kept from collapsing by a discontinuous thread of chitin coiled in a tight spiral. In many aquatic larvae the tracheal system is “ closed ” (i.e., does not com¬ municate with the exterior), and breathing is then carried on either through the skin, or through feathery or leaf¬ like processes which are prolific in tracheae and are known as tracheal gills (Figs. 164, 165). Reproduction.—The sexes are separate, but parthenogenesis 1 (irapOevo9 = virgin ; yevecris = manner of birth), or reproduction without male co-operation, is very common. The ducts of the generative glands open near the posterior end of the abdomen. In the female the vagina is often connected (1) with glands whose secretion either causes the eggs to cohere in masses, or envelops them in a definite capsule or nest; and (2) with spermathecce, or chambers where, after pairing, spermatozoa are stored for the postponed fertilisation of the eggs. In the male the vas deferens usually expands into a vesicle where the spermatozoa accumulate, and the ejaculatory duct is protrusible. Often the female is furnished with an ovipositor, and the male with paired claspers (Figs. 29, 81). The egg as a rule is large, and contains much food-yolk. In a few insects the egg is hatched within the mother, and
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