230 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 
but comparatively few forms depart either by way of excess or defect. Thus, if we leave 
out the Læmodipoda, all Podophthalmous and Edriophthalmous Crustacea have twenty 
somites, of which six are cephalic, eight thoracic, and six abdominal. In a very few 
Branchiopoda, and in Trilobita, there is more than the typical number of sonde but I 
believe that in all other Crustacea, where the number of somites is not twenty, it is less. 
The question of the typical number of somites in the body of the Insecta is one which 
has been much diseussed. But all the theories on this subject with which I am acquainted 
are, in my apprehension, vitiated by the mistaken view which their authors take of the 
composition of the Insect's head. Many seem to consider it to be a simple segment; while 
those who admit a multiplicity of segments, appear to be misled by the position of the 
eyes and antennæ, into regarding them as tergal appendages of the segments over whose 
sternal appendages they lie—as a kind of wings of the cephalic somites, in short. Again, 
it is supposed by many that the labrum and the lingua are the representatives of the ap- 
pendages of distinct somites, a conception which is at once negatived by the study of their 
development. 
Ås I have endeavoured to show, there are certainly five, and hypothetically six, somites 
in the head of Insecta; there are certainly at least three in the thorax; but the number 
in the abdomen has been as much disputed as the number in the head. Zaddach considers, 
as å general rule, ten to be the number of abdominal somites in Insect larvæ; Westwood 
and Newport enumerate eleven in some Hymenoptera, and this last is, I believe, the 
maximum number of somites which has yet been found in the abdomen. Now, if we 
assume the number of somites in the head to be six, the number in the thorax three, and 
the number in the abdomen eleven, we shall arrive at twenty as the maximum number of 
somites in the body of an Insect. 
This conclusion is in remarkably close accordance with the results obtained by M: 
Lacaze-Duthiers from his laborious and remarkable researches into the structure of the 
female genital apparatus of Insecta. M. Duthiers finds that the vulva always opens 
between the eighth and ninth abdominal somites, and that in Neuroptera, in Orthoptera, 
in most Hemiptera, and in Thysanura, three somites intervene between the vulva and ~ 
the anus, which is always placed at the very extremity of the body. There are thus — 
eleven abdominal somites, and, therefore, a total number of twenty, in these four orders. 
Some Hemiptera have the last abdominal somite abortive, and this appears to me to 
be the case in Aphis. In Coleoptera and Hymenoptera, the tenth and eleventh somites 
abort, nine only remaining : in Lepidoptera, finally, all three post-genital somites remain 
undeveloped. M. Lacaze-Duthiers’ researches tend to show that a fundamental unity 
prevails amidst those apparently most diverse apparatuses which we know as stings, 
borers, and ovipositors, and that they are always the result of a modification undergone 
by the ninth abdominal somite. 
I do not consider myself competent to give an opinion as to the details of the investi- 
gations to which I have just alluded, but I cannot refrain from expressing the belief that — 
the labours of future investigators will bring only a confirmation of their general 
accuracy. 
The only adult Insect, besides Aphis, which I have studied with sufficient care in refer- 
