224 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 



dually arises. These filaments, thickening and elongating, become the blades of the man- 

 dibles and maxilliB. The growth of the second maxiUse makes up, by its excess, for the 

 arrest of development of the mandibles and first maxUlEe ; for having already approximated, 

 their confluent or connate bases elongate as one great process, which extends back in the 

 middle line between the thoracic legs, until at length it attains more than half the length 

 of the body, and constitutes the well-known proboscidi form "labium" of the Aphis*. 



The thoracic members or legs have elongated so much, that their terminations are bent 

 inwards, to allow of their lying within the pseudo vitelline membrane. Their character- 

 istic subdivisions are indicated ; and the terminal claws are beginning to be formed. 



From this size up to that at which the larvae are born (PL XXXIX. fig. 4) (when they 

 are less than ^th of an inch in length), the principal changes are the following. The 

 ajDjiendages as compared to the body, and the latter as compared to the head, undergo great 

 elongation. The anterior pair of thoracic Kmbs and its somite, the prothorax, come into 

 veiy close contact with the head, so that the cervical separation becomes obsolete, or is only 

 indicated by a groove. The labrum and labium acquire their characteristic form and pro- 

 portions ; and the mandibular and maxillary setae elongate, and take their final position. 



The " siphons," so characteristic of the genus, appear as obtuse tubercles on the dorso- 

 lateral region of the fifth abdominal somite. The little larva exliibits unequivocal signs 

 of life, but still remains enclosed within its pseudovitelline membrane, to which another 

 transparent and structureless envelope, fitting the body of the larva and all its Umbs as 

 a loose glove fits the hand, seems to have added itself. This second coat is, in fact, 

 the embryonic integument, which is now being cast ; so that the creature must undergo 

 its first ecdysis either before, or immediately after, it is born. The head assumes its 

 normal proportions. The corneae become facetted ; and the pigment increases greatly in 

 amount, assviming the form of an oval deep-red patch. The clypeus and the procephalic 

 lobes unite, but readily give way when the head is crushed, and aUow of the exit of the 

 cerebral mass, which has in the meanwhUe been produced by a differentiation of the inner 

 substance of the procephalic lobes, just as the other ganglia are the product of the blasto- 

 derm of their somites. 



If the account of the development of the external organs of Aj}his which I have just 

 given be compared with the statements of KoUikert and ZaddachJ, it will be found that 

 there is a close correspondence in all essential respects between the embryogenic pheno- 

 mena of at least thi'ee orders of Insecta — the Semiptera, the Diptera, and the Neuro- 

 ptera. And, considering the universality of the law that the embryogenic processes of 

 members of the same class have a similar fundamental character, I do not doubt that 

 the development of all insects is, in its main features, a process similar to that described 

 in Aphis. 



* Zaddach considers, from his observations on Phryganea and other Insects, that the labium is the product, not of 

 confluent maxillae, but of an outgrowth of the sternum by which these are supported, the maxillae remaining as the 

 labial palpi. I do not deny that this may be the case in Aphis ; but I have been unable to find positive evidence of 

 the fact. 



f De prima Insectorum Genesi, 1842. 



J Die Entwickelung des Phryganiden-Eies, 1856. 



