lo ANATOMY AND MORPHOLOGY OF INSECTS. 



The Epiostracum consists of a single external lamina, which 

 differs considerably from the rest of the cuticle in the readiness 

 with which it takes stains. It is apparently quite structureless, 

 or is divided into hexagonal fields. 



The Endostracum is formed of numerous lamellae, except in the 

 hard plates of the exo-skeleton, which do not usually exhibit a 

 laminated structure. It is often divided by vertical planes into 

 hexagonal prisms, several corresponding to a single hypodermic 

 cell. In the imago, when it first emerges, the endostracum is 

 not usually present except where sclerites are already developed. 



Sclerites. — This term is applied to the denser cuticular struc- 

 tures which form the exo-skeleton. The hardness and thickness 

 of the several parts of the cuticle are by no means in direct 

 relation to each other. The denser plates are usually thinner 

 than the membranous parts of the cuticle between them. Thus 

 in the imago of the blow-fly the hardest dermal plates vary 

 from 10" to ao'' in thickness, whilst that of the intervening 

 integument in some parts is loo". The flexible portions of the 

 integument are usually transparent and colourless, whilst the 

 sclerites are deeply pigmented and opaque. 



Syndesmotio Membrane.^ — -The soft flexible cuticle between the 

 sclerites has been termed conjunctiva, but, as the articulations 

 which it forms are known as syndesmoses, I shall prefer the 

 term syndesmotic membrane. 



Scales, Setae, and Vibrissae are solid or hollow projections of the 

 cuticle. The large scales and setae are hollow, with a giant cell 

 beneath each, the trichogenic cell. It gives off a process which 

 fills the interior of the hair or scale. 



The large scales and setae are usually articulated with the 

 cuticle of the surface, which forms a kind of socket around the 

 opening or foramen, through which the process of the trichogenic 

 cell enters the seta. 



The smallest sets are solid, the larger ones are ribbed ; 

 some are deeply channelled on one side. 



The larger bristles (seta;) of the Diptera are remarkably 

 persistent in number and position throughout very large 

 families. Osten-Sacken regards their arrangement, chcetotaxy, 



