40 AN ACCOUNT OF BRITISH FLIES. 
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. 
* 
Fic. 7.—Balancer of a Cecid. 
The “balancer,” or “ haltere,” of the Cecidomyide (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 are 
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 fs 3 = cubatal 
3rd Me s; .== ** postical.” 
4th — « discoidal,” etc. 
+E Anatomy of the Blow- Fly,” p. 96. 
+ “Senses of Insects,” Sir J. Lubbock. 
