MUSCLES A XI) MSCERA OF THE IMAGO. 219 



and are termed ordinary striated muscles, or sometimes leg 

 muscles. The sixth group consists of muscles which have a 

 peculiar structure (see Chapter VIII.), and differ entirely from 

 the ordinary form in the Arthropoda ; indeed, I know no 

 similar muscles in any group of animals except insects. They 

 are usually known as wing-muscles, or muscles of flight. I 

 prefer the latter term, as there is a group of proper wing- 

 muscles, inserted into the sclerites of the wing-roots, which 

 differ in no respect from the ordinary muscles. Even the term 

 ' muscles of flight ' is open to objection, for, although they are 

 only found in winged insects, they are by no means the only 

 muscles of flight ; hence I term them great thoracic muscles. 



1. Muscles of the Head. — The muscles of the proboscis will be 

 described in another section. They are quite unlike the 

 muscles of manducatory insects, and it is, in my opinion, 

 impossible to draw any satisfactory morphological conclusion 

 from a comparison of the mouth muscles of the Diptera with 

 those of the Coleoptera or Orthoptera. The other muscles of 

 the head are referred to in relation to their functions. 



2. Inter- segmental Muscles. — I have used the term inter-seg- 

 mental muscles to designate those which form a layer imme- 

 diately beneath the hypodermis, and which unite the more 

 movable sclerites with each other. They apparently correspond 

 with the integumental muscles of the larva. These are little 

 developed in the head and thorax, but exist in a highly- 

 modified condition in the neck, prosternal region, and between 

 the thorax and abdomen, and form an almost continuous layer 

 on the inner surface of the hypodermis of the abdomen, so 

 similar to the subcutaneous muscles of the larva that the cor- 

 respondence between the two cannot be doubted. 



In the Coleoptera, and in other insects in which the segments 

 of the thorax are mobile on each other, such a layer of muscles 

 is found in a highly-modified condition in the thoracic cavity. 

 Thus Straus Durckheim [40] remarked that the great ventral 

 series of recti muscles, so characteristic of larvae, are repre- 

 sented successively in the imago of the Cockchafer, from before 

 backwards, by the retractors of the labium, the depressors of the 



