THE INSECT HEAD SNODGRASS 



447 



The Diplopoda and Chilopoda have likewise a composite head struc- 

 ture in which, so far as known, six segments are united in a cranium- 

 like head capsule. The insects have a highly indi^ddualized head 

 (fig. 2 B), in the composition of which there appear to be six seg- 

 ments, or possibly seven. In the Chelicerata, on the other hand, most 

 of the members have no distinct head, but this does not mean that 

 they preserve the primitive annelid condition; on the contrary, in 

 most of the Chelicerata cephalization has progressed so far that not 

 only the segments of the true head region, but also the segments of 

 the leg region of the body as well are all united into a large structure 

 known as the ccflialotliorax. 



The insect head. — A typical insect head (fig. 3 A) is a craniumlike 

 capsule supported on the body by the membranous neck {Cv). Its 

 anterior lateral and dorsal walls are strongly sclerotized, as is also 



Pre X IV V+Vl 



Ant E 



,Md 



~f 2Mx 



Lm iMx 



A 



FiGDBB 2. — Examples of different degrees of cephalizatiou in adult arthropods 

 A, head of a phyllopod crustacean, Eiibrnnchipiis, in which the principal part of 

 the head is the protocephalon (Pre), bearing the first antennae (lAnt), second 

 antennae (2Ant), compound eyes (E), and labrum {Lm), followed by a distinct 

 mandibular segment ilV), and the united maxillary segments (V-{-VI). D, 

 head of an apterygote insect, Machilis, in which are combined the protocephalon, 

 and the mandibular and maxillary segments. 



whatever there may be of a posterior wall surrounding the attach- 

 ment of the neck. The ventral wall (B) is so cramped between the 

 bases of the head appendages as to be scarcely recognized as the 

 floor of the head, and a large part of it is bulged out in the form of 

 a median lobe, which is known as the hypopharytix {Hphy). 



The exposed, hard-walled part of the head is often called the 

 " epicranium," but since different writers use this word with differ- 

 ent restrictions, the term craniuin will serve just as well, or better, in 

 a general sense to designate the skull-like part of the head in distinc- 

 tion to the appendicular parts and the ventral area between the bases 

 of the appendages. From the facial aspect of the head arise the 

 antennae (fig. 3, Ant)^ and on the sides are located the coTwpoimd 



