1885.] Colouring of Phytophagous Larvce. 277 



a broad white band. Examination proved that the white bands moved 

 with the contractions of the dorsal vessel, and showed the charac- 

 teristic structure of fatty tissue. Dissection and the use of the micro- 

 scope confirmed these conclusions. The rest of the larva was green 

 (due to its blood), with a slight black shading below the white bands. 

 The shading was the only part of the colour due to larval pigment. 

 The skin was so transparent that the tracheal system was quite 

 distinct, and the fat was removed from all the conspicuous surfaces 

 (except in the region of the bands) through which it would have been 

 visible, and would have added other white markings. But this tissue 

 was collected superficially along the median ventral line, producing a 

 ventral white band, which was always hidden, for the larva rests with 

 its ventral surface applied to the edge of a leaf. 



Last autumn (1884) I found that fat can also serve as the 

 vehicle for other colours. I was working at another of the Phyto- 

 phagous Hymenoptera — the larva of Croesus Septentrionalis, and I 

 found that the yellow colour round the lips of the seven median 

 ventral eversible glands is due to fat collected at these points, and 

 the yellow colour of the posterior segments also depends upon the 

 same cause, the colour being contained in the fat globules themselves. 



It is extremely likely that fat will prove to be a more important 

 factor in larval colouring than has been hitherto supposed. 



5. Changes in the Two Factors of Larval Colouration before Pupation. 



It has already been mentioned that the superficial derived pigments 

 of S. Ligustri become brown in the dorsal region, before pupation, 

 while the colour of the blood is unchanged. In D. V inula the whole 

 larva becomes reddish-brown, and in this case the green blood changes 

 to brownish-yellow. The true larval pigment also changes before 

 pupation, except when it is cuticular. Thus the larva of JE. Angularia, 

 becomes transparent by the disappearance of dark pigment, and the 

 green blood gives its colour to the larva. The green colour of the 

 blood is generally retained in the pupal state, and it is often of great 

 importance, as will be shown presently. The changes in colour 

 before pupation alluded to above, are for the protection of the larvae 

 in their new surroundings, against which their old colouration would be 

 very conspicuous. And this is therefore only true of a limited number 

 of larvae which wander in search of a place in which to pupate, or are 

 exposed against a new background (as E. Angularia). In all larva?, 

 however (except those entirely coloured by cuticular pigment), the 

 colours ultimately fade in the quiescent and contracted state imme- 

 diately preceding pupation. Even in green Noctua larvae coloured by 

 their blood, the fat is no longer withdrawn from the surface, and the 

 dpep green tint can only be seen in the interstices of this lobulated 

 tissue. 



