910 



INSECTA. 



scarcely to be detected. A similar condition of 

 parts exists in the heads of other Coleoptera. 

 The disappearance of one segment of the head 

 thus early in the larva state is in perfect accord- 

 ance with that progressive developement which 

 we know takes place in every part of the body, 

 and hence it was to be expected, that those 

 parts in which the changes first occur are those 

 which first entirely disappear. Hence the dis- 

 appearance of the fourth subsegment, which we 

 believe exists in the earliest stages of the larva, 

 and of which the antennae are the superior ap- 

 pendages. If we turn from this transitory 

 larva state of the insect to the permanent ver- 

 miform condition of the Annelida, the lower 

 Articulata, we find in the common Nereis a 

 condition of the head apparently analogous to 

 that of the vermiform larva. It is elongated 

 forwards, and formed of distinct segments, of 

 which the posterior ones, as in insects, support 

 the organs of vision. But these remarks on the 

 relations of the different parts of the head are 

 offered with much hesitation, because, in 

 Myriapods, which have usually been com- 

 pared with the larvae of insects, the form of the 

 head seems to be opposed to this mode of view- 

 ing its development in Articulata, since the 

 antennae and organs of vision are situated on 

 the most anterior part of a large and broad 

 shield, which has been considered the first 

 segment. But if this be correct, it will be diffi- 

 cult to explain the circumstance of the an- 

 tennas and corneas of hexapodous insects being 

 constantly situated posteriorly to the first three 

 sub-segments of the head, the labrum and clypei. 

 The appendages of the head, which form 

 part of the organs of manducation, correspond 

 in number to the number of sub-segments. 

 These parts are analogous to those which consti- 

 tute the organs of locomotion, when attached to 

 other segments of the body, as in Myriapoda, 

 and Crustacea. In the head of an insect the 

 mandibles are the proper appendages of the 

 fifth or basilar sub-segment, while a small but 

 freely articulated lobe, which sometimes exists, 

 as in some of the Brachelytra, at the inner 

 side of the mandible, appears to represent that 

 of the fourth. The stipes, or external portion 

 of the maxilla, which at its base articulates 

 with the cardo, and at its distal extremity is 

 connected with the palpus, seems to be the 

 proper appendage of the submentum, while 

 the inner portion of the maxilla, which origi- 

 nally appears to be a distinct part, and which 

 at its distal extremity supports the galea, 

 seems to be the proper appendage of the se- 

 cond sub-segment, and the labial palpi in like 

 manner represent those of the ligula or first. 

 It has been shewn by Savigny and others, that 

 these analogues of the organs of locomotion 

 undergo a very gradual change of form and use 

 in the different classes. In Myriapoda the ap- 

 pendages that belong to the basilar segment of 

 the head, which constitute the mandibles, are 

 greatly enlarged, and are directed forwards as 

 organs of prehension, like the chelate organs of 

 Crustacea and Arachnida, but are jointed and 

 retain the exact form of true legs. In insects 

 the mandibles are in like manner directed for- 



wards, and are placed above those of the pre- 

 ceding segments, but are compressed, and mate- 

 rially altered in size and shape, their terminal 

 portions, the tarsal joints, being undeveloped, 

 and the tibia alone enormously enlarged, con- 

 stituting the whole jaw or manducatory organ, 

 while the basilar joints, the femur and coxa, 

 are lost in the under surface of the segment, 

 with which they have become confluent. That 

 this is really the case is proved by the fact that 

 all the muscles that belong to these powerful 

 organs are attached to the basilar and postero- 

 lateral parts of the head, in the very situations 

 which they must have occupied had the organs 

 remained free for the purposes of locomotion 

 or prehension, as in Crustacea, Arachnida, or 

 Myriapoda. That this confluence of parts has 

 in reality taken place is further proved by the 

 circumstance, that the outlines of the portions 

 that become united with the skull are distinctly 

 marked in Lucanus cervus, and still more 

 clearly in that of the great Hydrous (fig. 369, 

 o). There is a remarkable illustration of the 

 principle upon which the change of form in 

 the adaptation of these organs to a new func- 

 tion depends, in that curious instance of mon- 

 strosity in an individual of Geotrupes sterco- 

 rarius, described in a former page (860), in 

 which the tibiae of the pro-thoracic legs rme 

 been arrested in their development, and are 

 lunated like the proper mandibles, the tarsi 

 being entirely absent. In a remarkable insect, 

 Onitis aygulus, to which our attention was 

 directed by Mr. Shuckard, there is a further 

 illustration of this principle, in the permanent 

 condition of the pro-thoracic legs of that spe- 

 cies, in which the tarsi are entirely absent, and 

 the tibiae are lunated and terminated each by a 

 sharp hook. There is also a similar condition 

 of the same parts in other species, O. Olivierii, 

 O.serripes, and O.chinemis and Apelles, while 

 in a species of a neighbouring genus, Bubos bison, 

 the tibiae are considerably narrower than in the 

 preceding, and approach much nearer in shape 

 to the instance of monstrosity in Geotrupes, 

 thus distinctly indicating, not only that the 

 form of parts depends either upon excessive or 

 deficient developement, but also that the abnor- 

 mal conditions occasionally met with in some 

 species are permanent normal conditions in 

 others. 



From the manner in which the appendages 

 of the cranial sub-segments are arranged to form 

 the parts of the mouth, it necessarily follows 

 that the most posterior pair, the mandibles, 

 are carried upwards, and become the superior 

 lateral organs ; while the maxillae obtain the 

 next place beneath them, and the whole are 

 covered in by the inferior arches of their re- 

 spective sub-segments, which constitute the 

 labium. 



In all insects, the whole of the parts of the 

 head in the perfect individual exist in the head 

 of the larva, the changes which take place 

 being only those of size and relative position. 

 When the head of the larva is smaller than that 

 of the future imago, as in the Hymenoptera, 

 its increase of size j ust before the insect changes 

 into a nymph, and when a great portion of the 



