THE BUTTERFLY: INTERNAL ORGANS. 49 



sac Jintl !it the interior base of the uiaxilhiry canal, i.s the opening of tlie sal- 

 ivary duct, which is a capillary flexible tube, running backward for only a 

 short distance before dividing- into two ducts, passing insensibly into glands 

 of a similar form which are at least half as long as the body, but by reason 

 of the many convolutions as they run beside the oesophagus extending only 

 as far as the base of the al)domen. 



As in the early stages of tlie insect's life, the mal[)higian vessels are com- 

 posed of three filiform crinkled tubes on each side of the body, their con- 

 volutions overlying the stomach, the ends free, uniting just before entering 

 the chylific ventricule, first one pair and then tlie third, to form a voy short 

 {•anal. 



Respiratory system. .Vccording to Dufour the tracheal vessels of 

 butterflies are purely tubulai', luuing none of the vescicular expansions 

 characteristic of the Sphingidae and some other moths, which partake of 

 nourishment Avhile hoAcring before a flower. The general plan is the same 

 as in the earlier stages, only here the organs are mnch less bulky, being re- 

 dnced to exceedingly slender vessels and branching tubes. "The very 

 short main trunk into which the stigmata open soon divides into branches, 

 which run to the special organ to be aerated and there often branch abruptly 

 into a great numl)er of fine tubes . . . The stigmata of the first pair lie in 

 the sides of the prothorax behind the prothoracic lobes" (Burgess) . Those of 

 the second pair, not mentioned by Burgess, nor indeed, we believe, by any 

 writer besides Burmeister,* lie just in front of the base of the hind wings, 

 concealed in the suture between the mesothorax and metathorax, at the 

 furtliest advance of the latter; they probably belong to the mesothorax, as 

 they and the derivative tracheae adhere to it on forcible rupture of these 

 parts. As Burmeister remarks, the [presence of such a pair is snrprising, 

 because no spiracles are present here in the caterpillar, though, as we have 

 seen, a spreading bunch of tracheae arise from the longitudinal canal where 

 the spiracle should be expected. Succeeding pairs of stigmata are situated 

 in the pleurae of the first seven abdominal somites, the pair in the first seg- 

 ment l)eing rather lianl to find owing to the folds in the integument of its 

 sides. 



Circulatory system. The haemal or dorsal vessel, somethnes called 

 the heart, "is a small tube lying immediately under the dorsal wall of the 

 abdomen, and hung in this position by triangular muscular sheets (the 

 alary muscles) which are placed in pairs, apices inwards, on either side . . . 

 The walls . . . contain two sets of muscidar fibres running spirally in op])0- 

 site directions. Slight constrictions divide the heart into a number of 

 segments, corresponding to those of the abdomen. Each segment has 

 probably a pair of clefts for the entrance of the blood ... In the basal 



*Miiiot uiiil Burgoss recogiiizo here in llie moth, Alotia, ••an incons^picnous >tructui< 

 interstitial membrane in tlie cotton worm \vlii<]i is pcrliaiis a spirn<'le." 



