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 Lcemodipoda, all Podophthalmous and Edriophthahnous Crustacea have twenty 

 somites, of which sis are cephalic, eight thoracic, and six abdominal. In a very few 

 Branchiopoda, and iu Trilohita, there is more than the typical number of somites ; 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 discussed. But all the theories on this subject with which I am acqiiainted 

 are, in my apprehension, vitiated by the mistaken Adew 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 antennge, 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. 



As 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 a general rule, ten to be the nmnber of abdominal somites in Insect larvae ; Westwood 

 and Newport enumerate eleven in some Symenoptera, 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 Semiptera, and in Thijsanura, 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 Goleoptera 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 wUl bring only a confirmation of their general 

 accuracy. 



The only adult Insect, besides Aphis, which I have studied with sufficient care in refer- 



