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[Packard. 
in contrast with the subspherical shape and heteronomy of the 
same parts in the pupiparous Diptera, although the third thoracic 
segment is considerably larger than the second. Wagner also 
calls attention to the absence of the thoracic sternites, those 
of the meso- and metathorax being transformed into the ento- 
thorax, a peculiarity in which Siphonaptera differ from other 
insects. On the other hand, the prothoracic sternite is entirely 
atrophied, and there is a great development of the projjleura. 
There a])pear to be no episternites. Also the insects of this 
order are remarkable for their free epimera, and the very large 
coxae. 
The great development of the legs is of coarse an adaptive 
character, and this affects the great development of the pleura, 
Fig. 35. Month-parts of Melophagus ovinus ; ^, /jp, hypopliarynx ; Ibr, 
labriim ; mx', labium; lb, labella; sd, salivary duct; mx, maxilla; ft, ful- 
crum tube; fl, lower fulcrum plate; If, lateral continuation of the lower 
fulcrum plate; B, oe, oesophagus; t, teeth. — After ilufjgenhcrfj. 
and atropliy of the sternites, th5 latter being most developed in 
winged and cursorial insects. "While," says Wagner, '-in most 
springing insects the femora are most developed, in the fleas the 
coxae are as large or larger than the femora." "In the former 
case we see that those muscles which are to play the chief part in 
springing are situated in the femora, where they are attached to 
the walls of the femora, and from there are continued as tendon- 
like structures into the tibiae. While also in this way in other 
insects at the instant of springing the femoio-tibial joint is 
extended, in the fleas tlie coxa is drawn back from the thorax, 
and it extends the femoro-coxal joint. This difference exj^lains 
the flea's power of leaping, for no other insect can relatively to 
its size leap so high. That general mechanical law, according to 
