1G 
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 ; conse¬ 
quently, they do not receive the ox} T gen directly from 
the air, but from the water in which they live, in this 
w'ay reminding us of the aquatic respiration of fishes. 
In a small Ephemeral insect (Chloeon dimidiatum), 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 antennae 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 
