Xo. 6J:] DIPTERA OF CONNECTICUT I MORPHOLOGY 61 



It is likewise advisable to use only the designation axillary lobe 

 for the posterior lobe of the winjr {olo of Fig. 7,1), as was suggested 

 I by Haliday (1839. Annals Nat. Hist., Vol. 3, p. 2i9) and WaU^er, who, 

 according to Osten-Sacken (189G, p. 288), probably follow Stenham- 

 mar in this matter. This usage is in harmony with that of entomol- 

 ogists in general, who apply the designation axillary lobe to a homol- 

 ogous structure in the wings of Hymenoptera and other insects. On 

 the other hand, the application of the term "alula" to the axillary lobe 

 is at variance with general entomological usage, because the term alula 

 is applied to a lobe homologous with the calypteres, not the axillary 

 lobe, in insects other than the Diptera (e. g., the alula of hydrophilid 

 beetles, etc.). It is likewise most confusing to have some dipterists 

 follow Westwood, Walker, Haliday, and others, in referring to the 

 calypteres as the alulae, while other dipterists follow Loew in refer- 

 ring to the axillary lobe as the alula ; and the simplest and most logi- 

 cal way out of the difficulty is to employ only the terms calypteres 

 and axillary lobe for the structures so designated here. 



7. The halteres 



Although Latreille and Kirby and Spence (see also the more re- 

 \ cent discussion by Feuerborn, 1921) considered that the halteres repre- 

 sent abdominal structures, there is no longer any doubt that they are 

 I the greatly modified and reduced metathoracic wings. In the larval 

 [ stages they are formed from wing-buds in the dorsal region of the 

 i metathorax, which are homologous with the mesothoracic wing-buds 

 jj giving rise to the wings of the adult Diptera. In the pupal stages of 

 j; primitive Diptera the halteres are enclosed in metathoracic wing cases, 

 [ which are borne along the sides of the metanotum exactly as the wing 

 f cases of the mesonotum are borne. In the adults of such primitive 

 Diptera as the tipulid Holonma grandis (see Snodgrass, 1909, Fig. 

 210, Plate 69), the venation, axillary cord, basal articulation, axillary 

 sclerites, tegula, etc., of a true wing are exhibited in the basal region 

 of the halter; and in mutations of Drosopliila described by Morgan 

 (see Imms, 1929, Fig. 566, p. 602), the halteres have the form of 

 j small wings with a clearly recognizable venation, thus clearly proving 

 ' that the halteres are the modified metathoracic wings. 



A typical halter consists of a broader basal portion called the 

 -cabellum, a slender stalk called the pedicel or petiole (for which the 

 term "midhalter" would be preferable, since the term pedicel refers 

 to the second segment of the antenna, w^hile the term petiole is us- 

 ually applied to the constricted basal portion of the abdomen of cer- 

 tain Diptera and Hymenoptera), and a knob-like distal portion called 

 fi the capitellum. The halter is quite mobile, being operated by four 

 I muscles attached to its basal region, and it is capable of rapid vibra- 

 • 1 tion. It contains blood and a fine branch of the tracheal system, and 

 is innervated by one of the largest of the thoracic nerves. 



The halter is an extremely sensitive organ, and its extirpation 

 eAidently administers a very severe shock to the insect. The experi- 

 ments of Weinland (1891) and others indicate that the halteres are 

 •■Inilancing" organs and serve to coordinate tlie movements of flight; 



