14 CONNECTICUT GEOL. AND NAT. HIST. SURVEY [Bull. 



In the richardiine fly Gnatho-plasma infestans, shown in Fig. 3, F, 

 the region of the head just below each of the eyes becomes prolonged 

 to form two horn-like projections which give the name "stag-flies" to 

 these insects ; and in the lauxaniine genus Monocera and siniilar forms 

 figured by de Meijere (1916a), other projections of the anterior region 

 of the head give it a very peculiar appearance in these insects.* 



As might be expected, the heads of the Diptera which live as 

 •ectoparasites on bats and other hosts, are frequently greatly modified, 

 along with the rest of the bod}-, in adaptation to the parasitic mode 

 of existence. Thus, in the nycteribiid shown in Fig. 7, B, the small, 

 narrow, pyriform head is laid back in a groove of the thorax of this 

 curious insect. The head of the streblid Megistopoda (or Pterellip- 

 sis) figured by Williston (1908, p. 384) is very irregular in shape, with 

 protruding prominences which give it a most unusual appearance, 

 while in other streblicls, such as the one shown in Fig. 7, F and G, the 

 head is greatly flattened dorso-ventrally, its surface is divided up into 

 irregular patches, and it is bordered posteriorly by a ctenidium, or 

 dense row of stout, backwardly-projecting, black bristles. 



In the parasitic forms, there is a marked tendency toward a re- 

 duction of the compound eyes, while in such forms as the pipunculids, 

 cyrtids, etc., the compound eyes occupy most of the area of the head 

 capsule, and their development greatly affects the general appearance 

 of the head. The eyes are usually dichoptic, or separated from each 

 other, but in some Diptera they are holoptic, or contiguous, partic- 

 ularly in the male sex. When the eyes become approximated, they 

 may meet above the bases of the antennae, and separate them widely 

 from the ocelli, as in Oncodes glhhosus (Fig. 1, D), or the eyes may 

 meet below the bases of the antennae, and leave the antennae in the 

 neighborhood of the ocelli, as in Acrocera gJohulus (Fig. 1, E). Oth- 

 er modifications of the eyes which do not have such a pronounced effect 

 upon the appearancci of the head will be described in the more detailed 

 discussion of these structures taken up later on. 



2. External landmarks of the head capsule 



Riley (1904) has very clearlj^ shown that even in such a primitive 

 insect as the roach Blatta^ the segments of the head region of the em- 

 bryo fuse so completely, to form the definitive head capsule of the adult 

 insect, that the original segmentation can no longer be distinguished 

 in the completed head capsule. The same is evidently true to an 

 even greater extent in such highly specialized insects as the Diptera 

 (in which the formation of the various head-sclerites has never been 

 eatisfactorily traced from the embryo to the adult stages) so that 

 Hendel (1928), de Meijere (1916), and others who interpret various 

 superficial markings on the head capsule of adult Diptera as the 

 boundary lines between the original segments of the head, are hardly 

 justified in so doing. Furthermore, manj' of the sutures and similar 

 markings of the head capsule of adult Di])tera do not correspond ex- 



*In the phytalmiid fly Phytalmodes shown In Fig. 627 of the "Classification of Insects" 

 by Brues and Melander (1932), the head has a very peculiar shape, with tlie reduced 

 proboscis borne on the concave ventral portion of the broad head in which the genal 

 regions beneath the eyes are prolonged downward on each side, to some extent. 



