224 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 
dually arises. These filaments, thickening and elongating, become the blades of the man- 
dibles and maxillæ. The growth of the second maxillæ makes up, by its excess, for the 
arrest of development of the mandibles and first maxillæ ; 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 thoracie members or legs have elongated so much, that their terminations are bent 
inwards, to allow of their lying within the pseudovitelline membrane. Their character- 
istie subdivisions are indicated; and the terminal claws are beginning to be formed. 
From this size up to that at which the larvæ are born (Pl. XXXIX. fig. 4) (when they 
are less than 45th of an inch in length), the principal changes are the following. The 
appendages as compared to the body, and the latter as compared to the head, undergo great 
elongation. The anterior pair of thoracic limbs and its somite, the prothorax, come into 
very elose 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 setæ 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 exhibits 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 limbs as 
a loose glove fits the hand, seems to have added itself. This second coat is, in fact, 
the embryonie 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 corneæ become facetted ; and the pigment increases greatly in 
amount, assuming 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 allow of the exit of the 
cerebral mass, which has in the meanwhile 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 Aphis which I have just 
given be compared with the statements of Külliker+ and Zaddacht, it will be found that 
there is a close correspondence in all essential respects between the embryogenic pheno- 
mena of at least three orders of Insecta—the Hemiptera, 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 
eben, of all insects is, in its main features, a process similar to that described 
(phis. 
* Zaddach considers, 
confluent maxillæ, 
labial palpi. I do 
the fact. 
T De prima Insectorum Genesi, 1842. 
I Die Entwickelung des Phryganiden-Eies, 1856. 
from his observations on Phryganea and other Insects, that the labium is the product, not of 
but of an outgrowth of the sternum by which these are supported, the maxillæ remaining a8 the 
not deny that this may be the case in Aphis; but I have been unable to find positive evidence of 
