40 AN A ceo UNT OF BRITISH FIIES. 



costal and cubital often close together, postical often forked.* Between 

 the cubital and postical is a vein-shaped fold. The little cross vein 

 is long and often so crooked that it looks like the beginning of the 

 cubital. 



The cross vein often found between the discoidal and postical is 

 wanting in the Cecids. 



Fig. 7. — Balancer of a Cecid. 



l^he.^^ balancer" or " //rt-Z/^r.?," of the Cecidomyidce (Fig. 7) : Lownet 

 divides the " balancers " or halteres of flies into three portions, viz., the 

 base {extremity which joins the body), pedicle (or stalk between the 

 two extremities), and globe (distal extremity). The base contains four 

 groups, each group composed of rows of vesicles (as Keller and 

 Lubbock name them), the number of rows varying in various kinds of 

 Diptera. The two upper rows or '* ridges " of vesicles, viz., those 

 nearest the pedicle, are generally arranged in the form of two half 

 cylinders, and the lower rows form a hemisphere. The grooves 

 between the rows bear curved hairs. Lowne thinks that the vesicles 

 are contained in sacculi, and that there are bright " corpuscles of 

 high refractive power" beneath them. In the blow-fly there aie 

 altogether nearly 1,000 vesicles in each halter. The globe contains 

 large vesicles of fluid. The nerve of the halter divides into several 

 branches in the base, and passing through the pedicle, ends in loops 

 and nerve cells in the globe. The vesicles are supposed by Keller, 

 Lubbock,! Lowne, and others, to be auditory rods, forming part of 

 the hearing apparatus of Diptera. Lowne (who describes a similar 

 structure in the sub-costal nervure of the blow-fly's wing, believed by 

 him to serve a similar purpose) remarks that, being beautifully 

 balanced by muscles, the halteres are not much affected by the vibra- 

 tions of the body, and are, therefore, well-fitted for the development 

 of auditory organs. 



Owing to the extreme smallness of the halter in a Cecid (the object 

 being almost invisible to the naked eye when detached from the 

 insect), we have found it very difficult to discern the rows of vesicles 



* 1st longitudinal vein= "subcostal." 

 2nd ,, ,, =" cubital." 



3rQ ,, ,, =" postical." 



4th ,, ,, = "discoidal," etc. 



+ " Anatomy of the Blow-Fly," p. 96. 



J " Senses of Insects," Sir J, Lubbock. 



