864 



INSECTA. 



bones in man. An instance of this occurs in 

 the upper surface of the cranium of every insect, 

 in the union of the clypeus posterior with the 

 epicranium. In another duplicature, one sur- 

 face of which is rendered concuve, and the cor- 

 responding one opposed to it convex, and 

 allowing of motion between them almost 

 wholly in one plane, we perceive the true 

 ginglymuid or hinge-like articulation ; while 

 the small intervening portion of tegument, by 

 means of which the margins of these surfaces 

 are connected, becomes thinned and atrophied, 

 and forms their proper connecting ligament. 

 Instances of tins kind of articulation occur also 

 in the head of most insects in the articulation 

 of the mandibles with the cranium, as well as 

 in the limbs of almost every species. Again, 

 when a portion of the tegument which covers 

 the developing organs of locomotion becomes 

 constricted at the base of the organ, that surface 

 of the duplicature which is nearest the body 

 forms a hollow or cup-shaped cavity, into 

 which the other surface of the duplicature, 

 rendered convex, is inserted, and in this way 

 a true enarthrodial or cotyloid articulation is 

 developed, the connecting ligament between 

 the two surfaces forming the internal ligament 

 of the joint, which is thus rendered capable 

 of most extensive rotation. The ligament tlms 

 formed in every instance is hollow, to allow a 

 passage for the muscles and other structures 

 of the limb. Examples of this kind of articu- 

 lation occur in the coxae or basial joints of the 

 legs, in the Cerambycida and Curculionida: . 

 Lastly, where the tegument is simply reflected 

 upon itself, and a sliding motion allowed of, 

 we have the simple squamous articulation. In 

 all cases the development of one portion of tegu- 

 ment takes place at the expense of another, as in 

 the development of the segments themselves, 

 and not by the introduction of a new element 

 in the composition of the part. In this manner, 

 in accordance with the law of centripetal devt- 

 lopment as pointed out by M. Serres in the 

 vertebrated classes, every part of the body is 

 formed in the so-called invertebrated. 



We thus recognise four distinct kinds of 

 articulation, although several more have been 

 described by Straus-Durckheim in his excellent 

 work on Melolontha,* but all of them appear 

 to be reducible to these primary ones. 



These principles will enable us to understand 

 the cause of the presence or absence of those 

 structures which form the internal skeleton, and 

 also the manner in which the limbs of the 

 imago are developed from the soft and uniform 

 body of the naked larva. They may also tend 

 to elucidate one of those hidden and mysterious 

 processes of nature by which the exterior orga- 

 nization of the queen or female inmate of the hive 

 is caused so materially to differ from that of the 

 so-called neuter or sterile female, influenced as 

 it is said to be in its whole system by the diffe- 

 rent quality of the food supplied to the larva 

 during the first few hours of its existence. 



According to the investigations of the most 

 careful observers, Savigny, Audouin, Mac- 



* Considerations, &c. p. 48 ct eq. 



leay, Kirby, Carus, Straus-Durckheim, New- 

 man, and others, every segment of the perfect 

 insect is made up of distinct parts, not 

 always separable from each other or developed 

 to the same extent, but existing primarily in all. 

 It is also believed that the head itself is formed 

 of two or more segments, but the exact number 

 which enter into its composition is yet a ques- 

 tion. So uncertain are the opinions held upon 

 this subject, that while Burmeister recognizes 

 only two segments, Carus and Audouin believe 

 there are three, Macleay and Newman four, 

 and Straus-Durckheim, even so many as seven. 

 These different conclusions of the most able in- 

 vestigators appear to have arisen chiefly from too 

 exclusive examinations of the head in perfect 

 insects, without reference to the corresponding 

 parts in the larvse. It is only by comparing the 

 distinctly indicated parts of the head in the per- 

 fect insect with similar ones in the larva that we 

 can hope to ascertain the exact number of seg- 

 ments of which it is composed. In the head of 

 the perfect insect there ought to be found some 

 traces of all the segments which exist in the 

 larva; of the same species, and in that of the 

 more perfectly developed larva; that undergo a 

 true metamorphosis, there ought in like manner 

 to be found the rudiments of all the segments in 

 the least perfectly developed. Now the com- 

 mon larva of the Dipterous insect, the maggot 

 of the flesh-fly, is one of the lowest forms we 

 have yet examined, and we have already seen 

 that its head appears to be formed of four, and 

 perhaps even of five segments. This is the 

 greatest number yet noticed in the head of the 

 larva of any species. If, therefore, we can 

 trace the like number in the head of a perfect 

 insect, we may fairly conclude that this is the 

 normal number of segments throughout the 

 class. The head of the great water-beetle, 

 Hydrous piceus, is remarkably well-fitted for 

 exemplifying the number of segments of which 

 the head is originally composed, the remains 

 of four of the segments being distinctly marked ; 

 and it also affords us a proof of the correctness 

 of the opinions advanced by Savigny and others, 

 that the organs of manducation are the proper 

 articulated members of distinct segments, and 

 are perfectly analogous to the proper organs of 

 locomotion. 



We shall first describe the parts of which 

 the head is composed, and then endeavour to 

 explain the manner in which these parts have 

 been developed from separate segments to form 

 the perfect cranium and its appendages. It 

 has hitherto been customary with naturalists to 

 designate the head the first segment of the 

 body, and as every change in the nomenclature 

 of a distinct part ought always to be avoided, 

 unless positively required, through fear of 

 creating confusion, we shall not deviate on the 

 present occasion from the established mode, 

 but when speaking of it as a whole shall con- 

 sider it the first segment, while the aggregation 

 of segments of which it is composed we shall 

 designate individually sub-segments, distin- 

 guishing them numerically in the order in 

 which they appear to exist in the earliest con- 

 dition of the foetal larva. 



