872 



INSECTA. 



a nymph, or pupa, the head is found to occupy 

 the anterior part of the second segment. The 

 true head of the hymenopterous larva, before its 

 changes have commenced, is in reality the 

 first segment; since, as remarked by Mr. 

 Westwood, it has not only the usual conforma- 

 tion of the head, but contains also the rudi- 

 ments of all the manducatory organs, and the 

 antennae. In addition to this, we may state 

 that before the larva has discontinued to feed, 

 and has begun to prepare itself for transforma- 

 tion, we have invariably found on dissection, 

 that the first cerebral mass, the supra-cesopha- 

 geal ganglion or brain is situated in the superior 

 part of the first segment, and the first sub- 

 cesophageal ganglion in the posterior part of the 

 inferior surface; so that it is not until after 

 the changes into the nymph state have com- 

 menced, beneath the skin of the larva, that the 

 head becomes so greatly enlarged as to en- 

 croach upon the second segment. 



Of the head. The head of a larva, excepting 

 in Dipterous insects as above noticed, is 

 usually of a rounded or oval figure, and of a 

 harder texture than other parts of the body. 

 At its inferior surface are situated the organs of 

 manducation, and at its lateral and anterior the 

 rudiments of the eyes and antennae. In all 

 true larvae it is divided longitudinally into two 

 halves, by a suture which extends from the 

 vertex or epicranium to the face, the front of 

 which is formed by a convex plate, the clypeus, 

 or shield (Jig. 359, 6). This is generally of a 



Fig. 359. 



Head of larva of Athalia centifoliee. 

 a, the epicranium ;'&, the clypeus ; c, labrum ; d, the 

 mandibles ; e, maxillae and palpi ; f, [the labium and 

 labial palpi. (Newport, Prize Essay.) 



semicircular, or a quadrangular form, but varies 

 considerably in different species. Immediately 

 beneath this plate is situated another, the 

 labrum or upper lip (c). This also is of an 

 elongated, quadrangular, and sometimes heart- 

 shaped form, and constitutes the anterior 

 boundary of the mouth. Beneath this plate are 

 a pair of strong horny jaws, mandibula (d), 

 which are in general'thick, curved, and strongly 

 indented or toothed, and are placed one on each 

 side of the head. Beneath these are a pair of 

 lesser jaws, maxilla (e), placed in a similar 

 manner, and with the mandibles form the 

 lateral boundaries of the mouth. The maxillse 

 are soft, membranaceous and adapted for 

 holding, rather than for comminuting the food 

 like the mandibles. They are in general also 

 furnished, as in the larva of Athalia, with two 



other jointed organs, palpi or feelers which are 

 employed by the insect entirely as tactors. 

 Behind these parts is situated a second trans- 

 verse plate, the labium (/), or inferior lip, 

 which bounds the posterior part of the mouth. 

 This also, like the maxilla, is furnished with a 

 pair of jointed palpi. The motions of the man- 

 dibles and maxillae differ from those of the 

 jaws in vertebrated animals, being always from 

 side to side, and meeting, or passing across 

 each other like the blades of a pair of scissors. 

 Besides these parts, there is in many larvae a 

 projecting papilla situated within the mouth 

 upon the soft membrane of the labium. This 

 is conical and jointed, and is called by Messrs. 

 Kirby and Spence the spinneret. It is the 

 common excretory duct of the glands which 

 secrete the materials with which the insect spins 

 its coccoon, previously to undergoing its trans- 

 formations. In all larvae the antenna (g) are 

 but slightly developed. They are situated a 

 little above the base of the mandibles, on each 

 side of the clypeus. and are of a conical form, 

 jointed, and usually terminating in a point. 

 In some species they are three, but rarely 

 more than five-jointed. The eyes in all larvae 

 are single, or sessile, and not compound, or 

 aggregated together, as in perfect insects. In 

 the pseudo-caterpillars, Tenthredinida, as in 

 Athalia centifolia, there is only one large stem- 

 ma on each side of the head (A), situated above 

 the antennae ; but in the true caterpillars, Lepi- 

 doptera, as in the Sphinx ligustri, there are 

 always six very minute ones, placed at a little 

 distance from each other, in the form of an 

 arc near the base of the mandibles and antennae, 

 at the lateral part of the head. In the apodal 

 hymenopterous larvae which constantly reside in 

 the dark, the oral apparatus is developed, but 

 the eyes are in general entirely absent. 



The form of the oral apparatus in the 

 maggots, or larvae of the Dipterous insects, 

 is entirely different from that of the insects we 

 have just described. In the larva of (Estrus 

 ovis instead of mandibles and maxillae crossing 

 each other transversely, the mouth is formed by 

 two fissures, the one anterior and longitudinal, 

 and the other posterior and transverse, the two 

 meeting each other in the form of the letter T 

 inverted thus j, (fig. 360). In the anterior 

 fissure (c) are situated two longitudinal power- 

 ful hooks, the mandibles (d} directed forwards 

 and downwards, and employed by the insect 

 both as organs of progression and nutrition. 

 At the base of these in the transverse fissure (e\ 

 are two other hooks, maxillae, of a similar des- 

 cription, directed both to the median line, but 

 jointed like the mandibles in Myriapoda, and 

 crossing each other like the mandibles of the 

 true larva. The hooks thus include between 

 them the cavity of the mouth, in this manner 

 adapted both for wounding and tearing as well 

 as suction, and it is curious to observe that we 

 have here in the larva of a true insect an ap- 

 proach to the vermiform type of the permanent 

 condition of the oral apparatus of the leech. In 

 the maggot of the larder-flies and flesh-flies 

 above alluded to, the mouth is formed some- 

 what differently. Behind the transverse hooks 



