4.52 MODERN CLASSIFICATION OF INSECTS. 
insects. The head is generally advanced and broad; the eyes gene- 
rally placed at the sides of the head, which enter the frontal cavity of 
the prothorax ; the ocelli are occasionally present, and two in num- 
ber; the antenne are ordinarily of moderate length, and composed of 
four or five joints*; they are mostly filiform, but sometimes have the 
last joint either thickened or very slender. In the aquatic species 
(where porrected antenne would be inconvenient) we find these organs 
greatly reduced in size, and received into cavities on the underside of 
the head. The mouth is of the promuscidate construction (see fig. 119. 
and 122.); the labium or canal being occasionaily greatly elongated, ex-. 
tending beneath the body, and either 3- or 4-jointed.- The four in- 
ternal delicate sete represent the mandibles and maxille ; the maxillary 
and labial palpi are obsoletet{ ; the labrum is distinct, triangular, and 
more or less elongated, closing upon the upper side of the labium at 
the base, when the setz enter the labial canal. Savigny has discovered 
also a minute elongate tongue, trifid in front, between the base of the 
mandibles and maxille, terminated behind by the pharynx in Nepa. In 
many species which prey upon other insects, as well as in such as reside 
beneath the bark of trees, the promuscis is very short. The body is or- 
dinarily rounded, oval, or more elongate and depressed. The prothorax 
is greatly developed (although it never covers the scutellum and wings), 
imitating in this respect, as well as in the great development of the 
mesothoracic scutellum, the order Coleoptera: the metathorax is of 
small extent ; on its under side it is furnished with two pores, which se- 
crete the fluid which gives to these insects so unpleasant a scent. The 
wings, when at rest, are carried horizontally upon the back, the mem- 
branous apex of one fore wing lying upon and crossing the same part 
of the other ; the anal area of the fore wings is narrow ; the basal part 
or corium of this pair of wings is often very thick, but occasionally it is 
sufficiently transparent to permit the nerves with which it is furnished 
to be perceived; these nerves, both in the corium and apical mem- 
brane, vary considerably in number and position in the different 
* In Hammatocerus, a genus of Brazilian Reduvii, the second joint is composed 
of a great number of minute articulations. 
+ The mode of insertion of the joints of the rostrum or Jabium necessarily in- 
fluences the mode of action of this organ. (See Spinola’s Essai, p. 26—.) 
¢ Savigny first discovered two minute oval-pieces attached to the third joint of 
‘the labium on the upper side in Nepa, and which he considers as the labial palpi, 
observing that if the sides of the labium were not turned up, these palpi would be 
at the under side of the joint, as in the Orthoptera. (Mém. An. Articul. pl. 4. f. 3.) 
