448 



DIPTERA 



" squama." ^ 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 tlie three lol)es 

 are alula, antisquama, squama. The 

 squama may be called " calypter " 

 when it covers the halter. 



The halteres — commonly called 



or poisers — are perhaps 



augie or lobe of the wiug ; b, the most characteristic of all the 



alula ; c, antisquama, squama Diptcrous structures, though they are 



alaris, or antitegula ; d, squama, , , . . c ,^ i^ • ^ 



absent m most oi the lew wmo-iess 



Fig. 216. — Parts at tlie base of the lialailCCrs 

 wiuo; iu CdUiphora. a, Anal 



squama thoracicalis, tegula, caly- 

 pter, or calyptron ; e, posterior 

 extremity (scutellum) of the 

 mesothorax ; /, scutum of meso- 

 thorax. 



forms of the Order. Outside the Dip- 

 tera similar organs appear to exist 

 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 ;"W"einland states ^ that certain canals existing in the interior 

 of the halter correspond to wing-nervures. The halter may l)e 

 described as a small rod-like body witli a head like a pin, tliis 

 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 u.se of the terms tegula and antitegula, 

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



2 Zcitschr. iviss. Zool. li. 1891, p. 55. 



