THE LlPE-TIISTOKY OP A LEPlDOl'TKliOUS INSECT. 243 



being central. The head and tail sometimes merely meet (in the flattest 

 eggs), sometimes slightly over-lap, whilst, in the dome-shaped eggs, the 

 head so over-lajjs, as to take very often a central position in the vertex 

 of the egg, forming a dark spot there, as in Acronycta, Skippers, and 

 many others. 



"The essential importance of tliis observation is, thnt it shows that 

 the embr^'onic position of the nervous system is the same in insects as 

 in vertebrates, and since it must therefore be identical also in the mature 

 animal, it follows that the venter of insects corresponds anatomically 

 with the dorsum of vertebi'ates, and vice versa. 



" As regards the actual change of position itself, and the position 

 afterwards taken by the larva, it seems to me that the important point 

 is, that the larva whilst still truly an embryo, that is, whilst still 

 attached to the yelk and egg structures, has the venter outwards, and 

 the dorsum towards the centre of the yelk or egg; but when it becomes 

 free it is no longer an embryo, it moves how it likes, and though the 

 position it takes up seems to be very uniform throughout each species and 

 even throughout whole families, still this has little, if any, embryological 

 significance. I have frequently seen larvae making this S-movement, 

 and though I have called it " creeping up its own ventral surface," it 

 goes on slowly, without any apparent voluntary or even muscular move- 

 ments, and appears to be due to the mere force of the growth and develop- 

 ment of the larva. Sometimes it seems as if the lengthening of the 

 larva led to the extremity of the tail impinging against the side of the egg- 

 shell and, instead of sliding onwards, being caught and bent up. It is 

 associated no doubt with the completion of the growth of the dorsal 

 surface previously defective by the large umbilical opening, and now 

 more abundant in proportion to the ventral surface. I should class it 

 therefore under Dr. Osborne's second heading rather than under his 

 fourth. It proceeds slowly and steadily, so that usually some progress 

 may be noted in five or ten minutes. 



" Very shortly after, what appear to be voluntary movements of 

 swallowing take place, the remainder of the yelk disappears, and 

 the remaining fluid is either absorbed by the larva through the skin, 

 or evaporates through the shell ; the trachea become visible by getting 

 filled with air, and the larva begins the process of eating through the 

 shell." 



Of the forward movement of the anal segment after its curvature, and 

 at the time when it is pushing back as it were the ventral surface of the 

 larva from its previous contact with the eggshell, Mr. Jeffrey (E.M.M., 

 xxiii., p. 173)* writes oi Bofya hyalinalis that on the seventh day at 5.20 

 a.m. the terminal segment had become ventrally incurved, gradually 

 increasing in length ; that in two hours more the incurvature had 

 perceptibly increased, and that soon after noon the anal segment had 

 reached to the first pair of thoracic legs and he " could plainly see it 

 advancing towards these legs and actually push them forwards in its 

 course," whilst at G p.m. tlie anal segment had reached quite to a level 

 with the eye-spots — pressing all the thoracic legs down in its course. 



* This reference is to one of the most complete accounts of the embryonic 

 development of a lepidopterous insect published in the British magazines. It 

 is impossible to quote it at length, and as it is easily available to all our readers 

 there is no real need. Dr. Osborne's article in Science Gossip for 1885 is also 

 well worth reading. 



