ITS OUTER SKELETON. 35 



Somites of the Cockroach. 



The exoskeleton of the Cockroach is divisible into about 

 seventeen segments, which are grouped into three regions, as 

 follows : 



TT -I f Procephalic lobes 



' ( Post-oral segments ... 3 



Thorax ... ... ... ... 3* 



Abdomen ... ... ... ... 11 



17 



It is a strong argument in favour of this estimate that many 

 Insects, at the time when segmentation first appears, possess 

 seventeen segments. f The procephalic lobes, from which a great 

 part of the head, including the antennae, is developed, are often 

 counted as an additional segment.^: 



The limbs, which in less specialised Arthropoda are carried 

 with great regularity on every segment of the body, are greatly 

 reduced in Insects. Those borne by the head are converted into 

 sensory and masticatory organs ; those on the abdomen are 

 either totally suppressed, or extremely modified, and only the 

 thoracic limbs remain capable of aiding in locomotion. 



The primitive structure of the Arthropod limb is adapted to 

 locomotion in water, and persists, with little modification, in 

 most Crustacea. Here we find in most of the appendages a 

 basal stalk (protopodite), often two-jointed, an inner terminal 

 branch (endopodite), and an outer terminal branch (exopodite), 

 each of the latter commonly consisting of several joints. It 

 does not appear that the appendages of Insects conform to the 

 biramous Crustacean type, though the ends of the maxillae are 

 often divided into an outer and an inner portion. 



We shall now proceed to describe, in some detail, the regions 

 of the body of the adult Cockroach. 



* Where the thorax apparently consists of four somites, as in some Hymenoptera, 

 Hemiptera, Coleoptera, and Lepidotera, the first abdominal segment has become 

 blended with it. 



t Balfour. Embryology, Vol. I., p. 337. 



E.g., by Graber. Insekten, Vol. II., p. 423. 



See, for example, Huxley on the Crayfish. 



