EXTERNAL ANATOMY OF INSECTS. 681 
socket, the joints terminating in a globular head, perfora- 
ted indeed for the transmission of muscles, &c., and 
which is received by a corresponding cavity of the ¢ibza 
or preceding joint, as may be seen in many weevils?. 
This admits of some rotatory motion.—The second is a 
mixed articulation between enarthrosis and ginglymus, 
when at the base of the ball a deep transverse channel 
receives a corresponding ridge of the bie or preceding 
joint: this may be found in Ruéela and probably many 
other Petalocerous beetles; and something very similar in 
the Predaceous ones.—The third kind is where there is 
little or no inosculation, and the joints are scarcely more 
than suspended : this takes place in the Orthoptera, Neu- 
roptera, &c.; but in Blatta and the hind-legs of Mantis 
there is some approach to the foregoing kinds. 
We are now to consider the number of joints of the 
tarsus, which varies considerably in the different Orders, 
and in one has been assumed as a clue for a subdivision 
of it into sections’, which, though not perfectly natural, 
is very convenient, and has been adopted by most modern 
Entomologists. In treating of this head, I shall use 
those denominations that have been employed by M. La- 
treille and others to express the variations of the num- 
ber of the tarsal joints in the Coleoptera, but shall apply 
them to insects in general. Insects in this view, there- 
fore, may be called pentamerous ; heteromerous ; tetra- 
merous ; trimerous ; dimerous ; oY monomerous. 
Pentamerous insects are those which have, five joints in 
all their tarsz. ‘This is the most universal, and may be 
called the natural number of these joints. More than 
* Pratre XXVI. Fic. 44, 46, 47. a. 
» By Geoffroy—JTist, Ins. i, 58. 
