170 
ANATOMY AND PHYSIOLOGY 
extremely disagreeable, but others are of an opposite 
description. In all cases, they probably serve more 
or less as a means of defence ; for it is likely that 
even those which are agreeable to our sensations, 
may prove repulsive to some kinds of enemies. This 
is rendered the more probable by the emission of 
these odours (except in a few instances) not being 
involuntary, but under the direct control of the indi- 
vidual, and they are seldom exhaled in great strength, 
in many cases not at all, except when it is disturbed- 
They appear also to serve as a guide to the sexes 
in discovering the place of each other’s retreat. 
The Coleoptera and Hemiptera afford the most re- 
markable instances of this property, and the latter 
have been ascertained by M. L§on Dufour to possess 
a special odoriferous organ. It is a purse-shaped 
bag, placed at the base of the abdomen, immediately 
beneath the viscera of digestion, secreting an oily 
fluid, which volatilises in the form of an invisible 
vapour, and escapes by pores in the sides of the meta- 
thorax, between the insertion of the middle and 
hinder legs. 
The only other substance which requires to be 
noticed under the head of secretions, is that very re- 
markable one which renders many kinds of insects 
luminous. It is not elaborated, however, by any set 
of vessels appropriated to the purpose, but consists 
of a minutely granular mass, analogous to the adipose 
tissue ; yellowish white, semitransparent, and com- 
pletely filled with tracheae. The investigations of 
M. Macaire show that this matter is essentially com- 
