24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I46 



the lower ends of the postoccipital sulcus; these grow transversely 

 and unite into a posterior tentorial bridge just within the occipital 

 foramen. A pair of anterior arms arises from the subgenal sulci 

 above the mandibles, or more frequently in the epistomal sulcus; 

 these grow posteriorly and unite with the posterior bridge. Pri- 

 marily the tentorium therefore comes to have the shape of the 

 Greek letter tt, but often the space between the arms is partly filled 

 by a central sclerotization giving the structure a resemblance to a 

 canopy suspended by four stays. It is probable that the structure 

 got its name from the latter situation because tentorium is the Latin 

 word for "tent." In some of the apterygote insects the anterior arms 

 are not yet united with the posterior bridge, and there is evidence 

 that these arms were primitively ventral head apodemes. On the 

 other hand, in some of the higher insects modifications take place 

 resulting in either enlargement or reduction of the anterior arms, 

 and in some cases an obliteration of the middle part of the bridge. 

 Such modifications, however, are clearly secondary. 



In immature insects the frontal region of the head is commonly 

 marked by an inverted Y-shaped line the stem of which continues 

 back over the vertex to the postoccipital margin. This line has long 

 been called the "epicranial suture," and supposed to be an important 

 structural feature of the head. It is now known, however, to be a 

 preformed line of weakness in the cuticle where the head wall will 

 split at ecdysis (Snodgrass, 1947). The line on the vertex is con- 

 tinuous with the splitting line on the back of the thorax, and the 

 arms diverge downward on the face at various angles from the 

 compound eyes to the clypeus. Only rarely is a remnant of this 

 ecdysial cleavage line preserved on the head of an adult insect. 



This account of an orthopteroid head will give the student a pic- 

 ture of the fundamental structure of the head in a pterygote insect. 

 Numerous modifications, however, will be found in other orders ac- 

 cording to the position the head takes on the neck and its adaptations 

 to different feeding habits on the part of the insects. The orthopteran 

 is said to be hypognathous because the mouthparts hang downward 

 from the lower margins of the cranium. In a prognathous beetle 

 with forwardly directed mouth parts, the change in position of the 

 head on the neck has involved various alterations in the head struc- 

 ture, particularly in its lower parts. A third type of head is 

 opisthognathous, as in the Hemiptera, in which the sucking beak 

 projects backward beneath the thorax and so causes adaptive changes 

 in the head structure. These and other derived types of head struc- 



