ANATOMY. 31 



lies very close to the anterior cross-vein. The three organs on the 

 distal section of the third vein vary somewhat in their positions relative 

 to each other and to the ends of this section of vein. The greatest 

 difference in this respect occurs in Aulacigaster leucopeza, where all 

 three organs are in the distal three-fifths of the section. In Aulaci- 

 gaster and in Curtonotum it is clear that the two organs near the junc- 

 tion of the first and costal veins really lie on the first vein. The only 

 really striking variation in these organs that I have found occurs in 

 Drosophila guttifera. This species has several pigmented areas on the 

 wings, and each of these includes either the junction of two veins or 

 else an organ of the type here under discussion. One of the latter is 

 on the posterior surface of the penultimate section of the fifth vein — 

 the only case in the group where I have found more than the usual 

 7 organs on the main part of the wing. 



The same 7 organs occur in the same general positions in the extra- 

 drosophiline genera Calliphora, Fucellia, Ensina, Chcetopsis, Camp- 

 toneura, Ochthiphila, Phytomyza, Diastata, Piophila, Sepsis, and 

 Mallochiella. Dolichopus, however, has only 5. 



Behind and just below the base of the wing proper there is a small 

 continuation of the membrane of the wing, lying close against the 

 surface of the thorax. This structure, called the calypter, squama, or 

 tegula, is fringed with fine hairs. 



HALTERES OR BALANCERS. 



The hind wings of the Diptera are represented by small organs, 

 known as balancers or halteres, that lie below and behind the bases 

 of the true wings (fig. 7, Ha). In the Drosophilinse they are whitish, 

 flask-shaped bodies. Each consists of three segments — a short basal 

 one, a somewhat larger roughly cylindrical middle piece, and a large 

 terminal pear-shaped one. 



There is a mutant race of Drosophila melanogaster, known as ''bi- 

 thorax," in which the metathorax resembles the mesothorax more or 

 less closely. As a part of this change, the halteres are often somewhat 

 wing-like. In some cases the black hairs that occur on the costal 

 margin of the wing are present, and a fairly definite wing-blade with a 

 few veins can be distinguished. Numerous types intermediate 

 between this stage and the normal haltere can be found. An exam- 

 ination of these specimens indicates that the constrictions between the 

 three segments of the haltere correspond, respectively, to the proximal 

 and distal costal breaks of the wing. 



The halteres of the Diptera generally probably contain organs that 

 enable the insect to orient to gravity when in flight. Hewett states 

 that in the house-fly they receive the largest of the thoracic nerves. 



