448 



DIPTERA 



CHAP. 



" 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 the three lobes 

 are alula, antisquama, squama. The 

 squama may be called " calj'pter " 

 when it covers the halter. 



The halteres — commonly called 

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



^^vm^ in Callinhora. «, Anal , , j_ ■ . • p n , 



angle or lobe of the wing ; b, the most characteristic 01 all the 

 alula ; c, antisquama, squama Dipterous structures, thouo'h they are 



alaris, or antitegula ; d, squama, -^ . ° . '' 



squamathoracicalis.tegula, caly- absent lU mOSt ot the lew WlUgleSS 



pter, or oalyptron ; c posterix^L- f^^,^-^^ ^f ^^J^g Qrder. Outside the Dip- 



extremity (scutellum) ot the ■•- 



mesothorax ; /, scutum ot meso- 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; Weinland states ^ that certain canals existing in the interior 

 of the halter correspond to wing-nervures. 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 sc[uama 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 



^ Osten Sacken, although making use of the terms tegula and antitegulaj 

 suggested the proi>riety of using squama and antisquama, as we have done. 



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



