INSECTA. 



877 



this insect, -which frequently remains thus 

 suspended more than ten or twenty hours be- 

 fore the transformation takes place. During 

 this time the four anterior segments of the 

 larva become greatly enlarged, and the seg- 

 ments assume a curved direction, occasioned 

 by the contraction, or shortening of the muscles 

 of the under surface of those segments, which 

 are repeatedly slowly extended and shortened, 

 as if the insect were in the act of laborious 

 respiration. This generally takes place at short 

 intervals during the two hours immediately pre- 

 ceding the change, and increases in frequency 

 as that period approaches. When the period 

 has arrived, the skin bursts along the dorsal 

 part of the third segment, or meso- thorax, and 

 is extended along the second and fourth, while 

 the coverings of the head separate into three 

 pieces. The insect then exerts itself to the 

 utmost to extend the fissure along the segments 

 of the abdomen, and in the meantime pressing 

 its body through the opening gradually with- 

 draws its antennae and legs, while the skin, by 

 successive contortions of the abdomen, is slip- 

 ped backwards and forced towards the extre- 

 mity of the body, just as a person would slip 

 off his glove or his stocking. The efforts of the 

 insect to get entirely rid of it are then very 

 great; it twirls itself in every direction in order 

 to burst the skin, and when it has exerted 

 itself in this manner for some time, twirls 

 itself swiftly, first in one direction, then in the 

 opposite, until at last the skin is broken through 

 and falls to the ground, or is forced to some 

 distance from it. The new pupa then hangs 

 for a few seconds at rest, but its change is not 

 yet completed. The legs and antennae, which 

 when withdrawn from the old skin were dis- 

 posed along the under surface of the body, are 

 yet separate, and do not adhere together as 

 they do a short time afterwards. The wings are 

 also separate and very small. In a few seconds 

 the pupa makes several slow but powerful 

 respiratory efforts ; during which the abdominal 

 segments become more contracted along their 

 under surface, and the wings are much en- 

 larged and extended along the lateral inferior 

 surface of the body, while a very transparent 

 fluid which facilitated the slipping off of the 

 skin, is now diffused among the limbs, and 

 when the pupa becomes quiet dries, and unites 

 the whole into one compact covering.* Ex- 

 actly the same thing occurs in the changes of 

 the sphinx. The limbs at first are all separate, 

 each one inclosed in its distinct sheath, but 

 within a very short period after the change they 

 become agglutinated together by the fluid 

 effused between them, and form the solid ex- 

 terior of the pupa case. The body of the insect 

 is now divided into three distinct regions, head, 

 thorax, and abdomen. The first step towards 

 this division is the contraction which takes place 

 in all the longitudinal and diagonal muscles of 

 the body, soon after the larva (Jig. 364) has ac- 

 quired its full size, by means of which each seg- 

 ment of the insect forms a slight intussusception, 

 the anterior margin of one segment being drawn 



* See also Entomologist's Text-book, p. 208. 



Fig. 364. 





Section of larva of Sphinx ligustri j 1 to 13, (dorsal 

 surf ace ) segments ', 1 to 12, (ventral surf ace ) ganglia ; 

 a, dorsal vessel ; b, its lateral muscle ; c d, oesophagus 

 and stomach ; e, ilium ; f, hepatic vessels ; g, caecum 

 coli ; h, colon and rectum j i, testis * * thoracic 

 legs ; t f t abdominal legs. Newport, Phil. Trans. 



under the posterior margin of the one which im- 

 mediately precedes it. This occurs in all the 

 segments which form the abdominal region of 

 the future moth, the nine posterior ones of the 

 larva. When the period of changing into the 

 pupa state has arrived, a much greater shortening 

 takes place in the muscles of the fifth and sixth 

 segments, and in some insects this is carried to so 

 great an extent that the whole body becomes con- 

 stricted in the fifth segment like an hour-glass, 

 and is thus divided into two distinct regions, 

 thorax and abdomen. The same change takes 

 place also in the muscles of the first and second 

 segment, by means of which the region of the 

 head is divided from that of the thorax (Jig. 365). 

 These duplicatures of the external covering are 

 carried to a greater extent on the under surface 

 of the first four segments than on the upper, 



