448 DIPTERA 



CHAP. 



" squama." l Those Muscidae in which the squama covers the' 

 halter like a hood are called" calypterate." In Fig. 216, we 



represent these structures, and in 

 the explanation have mentioned the 

 synonyms. The terms we think 

 most applicable to the three lobes 

 are alula, antisquama, squama. The 

 squama may be called " calypter " 

 when it covers the halter. 



The halteres commonly called 

 FIG. 216. Parts at the base of the balancers or poisers are perhaps 



wing in Callipkora. a, Anal , . 



angle or lobe of the wing; b, the most characteristic of all the 



alula ; c, antisquama, squama Diptei'OUS Structures, tllOUgh they Rl'6 

 alaris, or autitegula ; d, squama, . ^ J 



squama thoracicalis, tegula, caly- absent 111 most 01 the I6W WlllgleSS 



pter, or calyptron ; e posterior formg Q f the Orc l e r. Outside the Dip- 

 extremity (scutellum) or the 



mesothorax ; /, scutum of meso- tera similar organs appear to exist 

 thorax - only in male Coccidae. The pair of 



halteres is placed on the metathorax, one on each of the pleural 

 regions. They are believed to be the homologues of the hind 

 wings; Wei n land states 2 that certain canals existing in the interior 

 of the halter correspond to wiiig-iiervures. The halter may be 

 described as a small rod-like body with a head like a pin, this 

 terminal part being, however, rather variable in form. "We 

 have already stated that in many Diptera the squama forms a 

 hood, the position of which leads to the belief that it is an 

 important adjunct to the halter. Although the exact functions 

 of the halteres are far from clear, it is certain that they are 

 highly complex bodies, of extremely delicate structure : they are 

 doubtless sense-organs, possessing as they do, groups of papillae 

 on the exterior and a chordotonal organ (a structure for assist- 

 ing the perception of sound) in the basal part ; each halter 

 is provided with four muscles at the base, and can, like the 

 wings, execute most rapid vibrations. Seeing that they are the 

 homologues of wings, it is a remarkable fact that in no Diptera 

 are they replaced by wings, or by structures intermediate between 

 these two kinds of organs. 



Internal Structure. Information about the internal anatomy 



1 Osten Sacken, although making use of the terms tegula and antitegula, 

 suggested the propriety of using squama and antisquama, as we have done. 



2 Zeitschr. wiss. Zool. li. 1891, p. 55. 



