THE CLASS INSECTA, AND THE ORDER DIPTERA 27 



The salivary glands open on the hypopharynx, which in 

 some blood-sucking insects is produced into a long slender 

 tube, so that its secretion can be injected into the wound 

 made by the mouth-organs. With the alimentary canal may 

 be mentioned the long slender excretory tubes — Malpighian 

 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 larvK, 

 open at regular intervals on the sides of the thorax and 

 abdomen. The openings are known as stigmata, or 

 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 or 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. 

 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 communicate 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. 117, 118). 



Reproduction. — The sexes are separate, but parthenogenesis 

 (7ra/o0eVoj = virgin; ye'j/eo-i? = manner of birth), or reproduction 

 by unfertilised eggs, is very common. 



The ducts of the generative glands open at the posterior 

 end of the abdomen. In the female the vagina is often 

 connected (i) 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^ox chambers where, 

 after copulation, 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 



