14 ILLINOIS BIOLOGICAL MONOGRAPHS [184 



The tentorium (t) of the hypothetical head-capsule has three pairs 

 of invaginations, homologous with the invaginations in generalized in- 

 sects. The invaginations of the posterior arms (i.p) of the tentorium 

 are situated ventrad of the occipital foramen at the distal ends of chitin- 

 ized thickenings. The invaginations of the dorsal arms of the tento- 

 rium (i.d) are on the cephalic aspect near the antennae and adjacent 

 to the epicranial suture, while the invaginations of the anterior arms 

 of the tentorium (i.a) are situated in the epicranial suture and adjacent 

 to the dorso-lateral angles of the clypeus. 



The heads of all Diptera have a dorso-ventral extension, and in this 

 resp(^t resemble the heads of many generalized insects. Some of the 

 primary sutures, sclerites, and invaginations of the head of such an 

 insect are present in a number of the Nematocera and in a few of the 

 Brachycera. The hypothetical head-capsule has been constructed from 

 these forms. The heads of the Acalyptratae and the Calyptratae are 

 highly specialized by the modification, union, reduction, and membra- 

 nous development of parts, consequently very few if any primary char- 

 acters remain which can be homologized with these structures. The 

 membranous development of areas has been the most important process 

 of specialization. The stippled areas on the figures show the extent 

 of the membrane. The various parts of the head-capsule are discussed 

 individually and in the order in which they were described for the 

 hypothetical type. The heads of Diptera naturally fall into two groups 

 according to the presence or absence of a frontal suture (fr.s) and a 

 ptilinum (pt). The forms without a frontal suture are the more gen- 

 eralized. 



Epicranial Suture. — The epicranial suture of all insects originates 

 in the embryo. The stem of the suture on the dorso-meson represents 

 the line along which the paired parts of the head meet, while the arms 

 of the suture (a. e. s) represent the place of contact between the paired 

 sclerites of the head and the mesal unpaired sclerites. The epicranial 

 suture (e. s) of a hypothetical dipterous head corresponds to the above 

 description, and is homologous with the epicranial suture found 

 in the heads of generalized immature and adult insects of the more com- 

 mon orders. The following examples illustrate the homology between 

 the hypothetical type and other insects. The epicranial suture in the 

 larva of Corydalis, and in the generalized larvae of the Coleoptera, Lepi- 

 doptera, and certain Hymenoptera, is complete, and its two arms join 

 with the margins of the clypeus, as in the hypothetical type. 



The epicranial suture of the adults of the Orthoptera, Hemiptera^ 

 and Hymenoptera also resembles this suture in the hypothetical head, 

 providing the following interpretation of this suture is accepted. In 



