38 



some specialist, one may get a fair conception of the work already 

 accomplished in this branch of our science. Not that these men 

 have studied their subject as entomologists per se, but rather as 

 biologists in the broadest sense. It has been recognised for at least 

 a century and a half that a knowledge of the structure and functions 

 of the organs of insects afforded a valuable basis for the further 

 study of analogous organs in the higher animals. The broad 

 principles of insect anatomy and physiology were worked out in the 

 last and early part of the present century by Swammerdam, 

 Reaumur, Lyonet, Latreille, Cuvier, Kirby and Spence, Burmeister, 

 and more recently by Westwood, Huxley, Graber, Brauer, and a 

 host of eminent biologists, and their work can of course be con- 

 sulted in the ordinary way. Much yet remains, however, to be 

 done even in this direction, especially in matters of detail. 



There is no need, therefore, apart from the impossibility, to deal 

 with this branch of entomological science, or even, indeed, one 

 branch of it in detail ; and our remarks, put into the form of a survey 

 of a few of the leading facts, must be exceedingly brief. 



In all insects the skin consists of a layer of epithelial cells called 

 the hypodermis, which secretes the cuticle, the latter being of 

 varying thickness, flexibility, and durability in different insects. The 

 cuticle becomes hardened by the presence of a substance called 

 chitin, whilst the joints or portions of the cuticle, where movement 

 is possible, remain thin and flexible. This outer covering, then, 

 whilst not interfering with the freedom of movement, forms a more 

 or less solid crust, and is a permanent protection to the soft organs 

 within ; at the same time it makes a solid base for the attachment 

 of the muscles. Chitin is a most remarkable substance, practically 

 insoluble under chemical reagents, is rapidly deposited at the end of 

 embryonic life, and quickly hardens on exposure to air. There are 

 numerous pores or canals passing through the cuticle ; some of 

 these carry off the secretions from the dermal cells ; through others 

 hairs or setae pass, whilst others are even supposed to have some 

 functional purpose in the aeration of the tissues directly beneath the 

 chitinous covering. 



The segments themselves are simply thickenings of a continuous 

 cylinder of skin, and are not independent individual rings or 

 segments. The segmentation of the skeleton, too, is correlated 

 with the serial arrangement of many of the organs, e. g. the ganglia 

 of the nervous system, the ostia of the dorsal vessel, the outer 

 openings of the respiratory tubes, &c. In the unjointed worms the 

 body forms a single but flexible lever. In the earthworm the body- 

 wall is divided into a number of somewhat hardened segments 

 joined by flexible intersegmental membranes, so that each section 

 can be moved independently. So similar, in the main, is the struc- 

 ture of many insects, especially the larval forms, that one might well 

 suppose that this metameric structure of worms and insects has been 

 inherited from a common ancestor. The peculiar segmented con- 



