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 o 

 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 disturb 

 They appear also to serve as a guide to the sexe 

 in discovering the place of each other's ret: 

 The Coleoptera and Hemiptera afford the most 

 markable instances of this property, and the latter 

 have been ascertained by M. Leon Dufour to possess 

 a special odoriferous organ. It is a purse-shapec 

 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 anc 

 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 insect 

 luminous. It is not elaborated, however, by any se 

 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 o 

 M. Macaire show that this matter is essentially com 



