18 BRITISH BUTTERFLIES. 



CHAPTER VII. 



EXTERNAL STRUCTURE OF THE BUTTERFLY LARVA. 



The butterfly larva is composed of a head, thorax and abdomen. 

 The Last two form a cylindrical tube slightly drawn in by a series 

 of constrictions following one after the other, and dividing it into 

 segments, separated by the drawn-in parts or incisions. The head of 

 the larva appears to consist of four segments, in most cases, closely 

 welded together, but in some instances traces of division are here and 

 there noticeable. The newly-hatched larva of Pararge megaera also 

 shows a somewhat peculiar development, the last head- segment bearing 

 four typical trapezoidal tubercles, arranged as a trapezoid (as on the 

 body-segments), each carrying the usual hairs. The marks on the other 

 head-segments seem to have the same significance, and it affords 

 strong suggestion that the head-segments were originally ordinary 

 tubercle- and hair-bearing segments like those of the body. It is also 

 interesting to note that the newly-hatched larvae of Limenitis sibylla 

 and Eugonia polychloros have fleshy spikes on the head, which are 

 probably of the same origin and significance as the spines of the thoracic 

 and abdominal segments. 



The three segments following the head are the thoracic segments, 

 and the ten following these comprise the abdomen. The 1st of the 

 thoracic segments is known as the prothorax, the 2nd as the meso- 

 thorax, and the 3rd the metathorax. Of the ten abdominal segments the 

 hindmost is known as the anal segment. There is often considerable 

 difference between the armature or clothing of the thoracic and 

 abdominal segments both as to the structure and arrangement. The 

 prothorax usually differs more from the meso- and metathorax than do 

 these from the abdominal segments ; the 8th, 9th and 10th abdominal 

 segments are also generally considerably modified. The number of 

 subsegments, into which the segments are subdivided transversely, 

 ordinarily differs in the thoracic and abdominal segments, the prothorax 

 being usually greatly specialised and differentiated in this as well as 

 in other details. The peculiar structure of the neck-like prothorax 

 of the Urbicolids is very remarkable and exactly opposite to that 

 of the swollen prothorax of the Huralids or Lycamids, into which the 

 head is freely retractile. There is also a remarkable swollen vesicle, 

 which is placed (usually in a transverse slit) below the 1st thoracic 

 segment, just in advance of the first pair of legs. It is know T n 

 as the chin- gland. This structure, butterfly larv;e have in 

 common with those of the Notodontides, Noctuides, and a few 

 closely allied superfamilies. Again, the spines or hair- bristles 

 arising from the skin are usually arranged in longitudinal 

 rows, which have a regular position on each abdominal seg- 

 ment; yet, whatever the arrangement on the abdomen, it will 

 usually be found that this either stops altogether, or changes in 

 direction, as soon as the thoracic segments arc reached. Tin- thoracic 

 segments bear the true legs, and the meso- and metathorax, under 

 which the structures that will form the future wings begin to develop 

 in the earliest larval life, have no spiracles, although the prothorax 

 carries one on either side. The abdominal segments that are probably 



