84 MEDICAL ENTOMOLOGY 



of sense organs. They are believed to function in regulating the flight 

 and balance of the insect. 



In the calypterate Diptera the halteres are covered by the squamae. 

 Except for this they do not provide characters of use in classification. 



THE LEGS 



The legs are six-jointed, the joints being named from above down- 

 wards as follows : coxa, trochanter, femur, tibia, tarsus and metatarsus. 

 They are always fairly long, and may be, as in the 



The Legs mosquitoes, very long and slender. In the purely 



(Plate XV, figs. 5, f 



5 and 7) parasitic forms, such as Htppobosca and Melophagus, 



they are shorter but very stout. The relative propor- 

 tions of the different joints does not vary much in the order. The 

 coxa, by which the leg is articulated to the thorax, is short, generally 

 oval in shape, and stout. The trochanter is a small joint which connects 

 the coxa with the femur. The femur and tibia are long and generally 

 cylindrical. The tarsus consists of five joints, of which the last four, 

 the tarsus proper, are short, not much greater in length than in breadth ; 

 the first joint is often longer than the rest, and is sometimes called the 

 metatarsus. As Colonel Alcock has pointed out, the use of the word 

 metatarsus in this connection is hardly accurate, as the joint referred to 

 is proximal to the tarsus, not distal to it. 



The joints of the legs may be coloured in various ways, and are 

 frequently banded. They may have on them prominent spines or hairs, 

 which may furnish characters useful in distinguishing species. 



The foot of a fly consists of a pair of claws, with a pair of pads 

 between them. The claws or ungues may be very delicate, as in the 

 mosquitoes, or extremely strong and serrated, as in the Pupipara. They 

 may be variously toothed and serrated, and thus yield useful distinguish- 

 ing characters. All the claws may be alike, or those of the fore legs may 

 differ from the other two pairs. 



Between the claws there are other structures the form of which 

 furnishes diagnostic features. These are the pulvilli and the empodium. 

 The former are bilateral, and consist of small elongate pads of fine 

 glandular hairs, which secrete a sticky substance, said to be used by the 

 fly when crawling on a slippery surface. One of these pads arises near 

 the base of each claw, and the empodium lies between them. Its form 

 varies a good deal in the different families ; it may consist of only a few 

 moderately stout hairs, with a complex arrangement of nerve cells at its 

 base, or it may consist of many hairs like those of the pulvilli, from which 



