28 ILLINOIS BIOLOGICAL MONOGRAPHS [198 



and connect with the arms of the epicranial suture (a. e. s). The in- 

 vaginations of the anterior arms are situated near the ventral ends of 

 the arms of the epicranial suture. The invaginations on each lateral 

 half of the head are joined together by the arms of the epicranial suture 

 and resemble the hypothetical type. Two pairs of invaginations are also 

 present on the cephalic aspect of Simulium (Fig. 2 and 3), but in this 

 genus they are not as prominent as in Tabanus. They are situated on 

 the vertex (v), adjacent to the compound eyes. In the female the arms 

 of the epicranial suture are well defined and the invaginations are 

 closely adjacent to them, while in the male the sutures are wanting. 

 Tabanus and Simulium are the only forms figured which show two 

 distinct pairs of invaginations on the cephalic aspect. All other genera 

 have only one pair and these are of two types. They are either long 

 and slit-like or they resemble small pits or darkened spots on the ectal 

 surface. The long slit-like invaginations found in Leptis (Fig. 35), 

 Psilocephala (Fig. 36), Platypeza (Fig. 32), Scenopinus (Fig. 41), 

 Exoprosopa (Fig. 29), Stratiomyia (Fig. 27), Mydas (Fig. 30), Erista- 

 lis (Fig. 25), and other genera have a special significance which will 

 be more fully discussed later. The small, pit-like invaginations are 

 present in the Nematocera and in Pipunculus (Fig. 38) and Empis 

 (Fig. 40). These are situated on the chitinized area of the vertex; or 

 on the fronto-clypeus, adjacent to the arms of the epicranial suture and 

 usually close to the compound eyes. Their position and structure indi- 

 cate that they are the invaginations of the anterior arms of the tento- 

 rium. In a few of the genera of the Orthorrhapha and in some others, 

 as Lonchoptera (Fig. 37), Tipula (Fig. 18), and Aphiochaeta (Fig. 

 31), no invaginations are present on the cephalic aspect of the head. 



One pair of invaginations, that for the posterior arms (i. p) of 

 the tentorium, is present on the caudal aspect of the heads of all genera 

 examined except Oncodes (Fig. 105), Olfersia (Fig. 139), Tipula (Fig. 

 95), and perhaps a few species of other genera in which it is difficult 

 to be sure of their presence. These invaginations in Bibiocephala (Fig. 

 83), Trichocera (Fig. 76), Dixa (Fig. 79), Rhyphus (Fig. 80), Sciara 

 (Fig. 81), Psychoda (Fig. 82), Rhabdophaga (Fig. 86), Chironomus 

 (Fig. 88), Bittacomorpha (Fig. 85), Mycetophila (Fig. 87), and Myce- 

 tobia (Fig. 90) are decidedly ventrad of the occipital foramen and 

 adjacent to the proximal ends of the maxillae. They are connected 

 with the lateral margins of the occipital foramen by means of the para- 

 postgenal thickenings except in Chironomus and Trichocera. The above- 

 named forms closely resemble the hypothetical type. In a few genera 

 of the Nematocera, such as Psorophora (Fig. 96) and Simulium (Fig. 

 77), the invaginations are adjacent to the occipital foramen. This 



