16 INTRODUCTION. 



openings, a pair of which is usually to be seen on each 

 side of the segments, excepting the head and the last 

 segment of the abdomen, the air gains admission into 

 the tracheal tubes. These openings, called spiracles or 

 stigmata, are often beautiful microscopic objects, some- 

 times possessing valves which open and shut like the 

 folding of a little door. The aquatic larvae of many of 

 the Orthoptera, Neuroptera, and Diptera, possess 

 tracheae which have no openings or spiracles ; conbc- 

 quently, they do not receive the oxygen directly from 

 the air, but from the water in wiiich they live, in this 

 way reminding us of the aquatic respiration of fishes. 

 In a small Ephemeral insect (Chloeon dimidiatiim), the 

 larva in its first three stages has no tracheae developed, 

 though subsequently it developes the tracheal gills. 



The nervous system of an insect, in its most charac- 

 teristic form, consists of a double cord which runs down 

 the central portion of the body, and unites a series of 

 nerve-knots or ganglia, as they are termed. Their 

 normal number may be supposed to be eleven ; three 

 for the head and thorax, and eight for the abdomen ; 

 but in point of fact they often fall short of this 

 number, some ganglia fusing with others, or becoming 

 abortive. From each of these ganglia or nerve-centres 

 various nerve filaments arise and are distributed to the 

 various organs. A nervous mass placed above the 

 oesophagus constitutes the insect's brain, from which the 

 nerves of the eyes and the antenna) are given off. 

 There is another nerve-mass just below the oesophagus, 

 "which unites with the brain-mass by a pair of nervous 

 filaments, and forms the nerve-collar. 



The sexes in insects are always distinct; there are no 



