EMBRYOLOGY OP A LEPIDOPTEROUS INSECT. 21 



the young larva is truly a larva, possessing no organic connection with 

 the other egg structures, and may no longer be regarded as an appen- 

 dage to the yelk-sac. The first use it makes of this liberty is to 

 assume the S or pot-hook shape, continuing until at length its 

 position is reversed, the dorsum being along the circumference of the 

 egg and the venter being central. The head and tail sometimes 

 merely meet (in the flattest eggs), sometimes slightly overlap, whilst 

 in the dome-shaped eggs, the head so overlaps as to take, very often, 

 a central position in the vertex of the egg, forming a dark spot there, 

 as in Acronycta, Callimorpha, Hesperids, and many others. The essen- 

 tial importance of this observation is that it shows that the em- 

 bryonic 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, ana- 

 tomically, with the dorsum of vertebrates and vice versa. Another 

 important point with regard to this movement is, that whilst the larva 

 is still truly an embryo, i.e., attached to the yelk and egg-structures, 

 it has the venter outwards, but when the embryo becomes free, it 

 moves as it likes, although this particular movement goes on so slowly, 

 and without any apparent voluntary or even muscular effort, that it 

 appears to be due to the mere force of the growth and development of 

 the larva. 



During all this time, the disappearance of yelk has been taking 

 place, but just when the embryo has attained its full growth, voluntary 

 efforts to swallow are apparent, and the remainder of the yelk dis- 

 appears. The remaining fluid is either absorbed by the larva through 

 the skin, or evaporates through the shell ; the tracheae become visible 

 by becoming filled with air, and the larva usually begins soon after- 

 wards to commence eating its way through the shell. 



It would appear from Jeffrey's observations that the tracheae come 

 rather suddenly into view, at the time that they are first distended 

 with air. He states that " the filling of the tracheae commenced in 

 the posterior segments, a sort of cloud gathering at the band where it 

 is close to the head and in a line with the eye." He says : " I saw an 

 apparently dark flood start from this spot, and, creeping along with a 

 spasmodic effort, filling the branches, in its course, till it reached the 

 head, and the whole of the tracheae became conspicuously visible on 

 that side of the body." 



The same observer describes how the dorsal vessel (heart) became 

 visible in an embryonic Botijs hyalinalis, on the tenth day after incu- 

 bation. The pulsations were at first (8 a.m.) very faint and feeble, 

 taking place somewhat irregularly at long intervals of 20 and even 

 30 seconds ; but, after a few hours, they became more distinct, 

 with shorter intervals between each beat, and became still more ac- 

 celerated by the evening of the same day. Two days afterwards, a 

 beautifully clear view of the heart and its action was obtained, the 

 pulsations being timed at 40 per minute, increasing to 60 a few 

 minutes before the larva escaped from the egg. 



The important part played by the blood-tissue in larval nutrition, 

 together with the supposition, entertained for many years by certain 

 eminent naturalists, that circulation of the blood did not take place in 



* Ent. Mo. May., vols. xxii. and xxiii. 



