NO. 2 THORACIC MUSCLES OF THE COCKROACH — CARBONELL 3 



not differ much from that described by Snodgrass (II, p. 54) ^ in the 

 Carolina grasshopper. 



The whole thorax of the cockroach, though not nearly as flexible 

 as the cervical region, has nevertheless a certain freedom of move- 

 ment, which is allowed by the partial desclerotization of its sternal 

 plates (III, p. 171). Responsible for the thoracic movement is a 

 complicated set of ventral muscles (fig. 54) probably working to- 

 gether with the dorsal oblique and longitudinal muscles and a few 

 small sterno-phragmal muscles. The function of each one of the 

 ventral muscles is rather obscure, and a written explanation would 

 add but little to what can be gathered by looking at the figures. 



Observing the living insect, two different kinds of movement can 

 be seen. One is an alteration of the relative position of the thoracic 

 segments, particularly noticeable in the prothorax, which the insect 

 moves to a certain extent on the horizontal plane. The second type of 

 movement is observable on the ventral side of the insect. It consists 

 in retractions of the different sclerites and alterations of their relative 

 positions. These movements of the thorax are probably helpful to the 

 insect when it progresses in narrow crevices. 



THE FORM OF THE THORAX AND MECHANISM OF THE LEGS 



The thorax of the cockroach differs in several respects from the 

 typical pterygote insect thorax as described by Snodgrass (I). The 

 differences are in the first place structural. The whole body of the 

 insect is depressed, apparently as an adaptation to living in crevices, 

 under stones, and crawling into the narrow hiding places where 

 blattids generally live. The three thoracic segments are as flat and 

 broad as the rest of the body, and the flat coxae, instead of extending 

 out from them as in a grasshopper, project from the thorax backward, 

 in a nearly horizontal position, underlapping the ventral part of the 

 following segments. This modifies the position of the leg, which 

 moves in a plane nearly parallel to the longitudinal axis of the body 

 instead of perpendicular to it. When the legs are at rest, the anterior 

 and posterior faces of the coxa, trochanter, etc., become respectively 

 ventral and dorsal. When the animal walks, the angle formed by the 

 coxae and the body increases slightly, more in the middle coxae than 

 in the hind ones, and more in the front than in the middle ones. Hold- 

 ing the living insect upside down between the fingers, it can be seen 

 that it is able even to move the coxae to a position perpendicular to 

 the axis of the body, the fore coxae having much more freedom of 



1 Roman numbers in parentheses refer to literature cited at the end of the work. 



