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 individual, 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 remarkable instances of this property, and the latter have been ascertained by M. Léon 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 metathorax, 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 remarkable 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 completely filled with tracheæ. The investigations of M. Macaire show that this matter is essentially com-
