192 ME. E, B. POULTON ON THE EXTERNAL 



aids in this part of the investigation. Not only is there the test of the spiracles, but 

 owing to the manner in which the pupa is formed beneath the larval skin, almost any 

 outgrowth of the surface of the latter leaves its impress upon the former. The scars 

 thus produced are merely incidental and of no morphological significance, but they are 

 extremely valuable as proofs of segmental identity. Other larval characters also appear 

 on the surface of the pupa ; their morphological meaning varies in the different cases. 



1. The Claspers. — The posterior or anal pair of claspers are generally distinct in the 

 pupa as convex cushion-like structures on each side of the anus. In certain individuals 

 they may even retain the relative size and appearance which are characteristic of the 

 larva (see Plate XX. figs. 8, 9, 10, showing different aspects of these parts in an 

 extreme variety of Smerintlms 2')opuU). While the posterior claspers are thus represented 

 by pupal structures and not merely by scars, the four anterior pairs only leave function - 

 less traces upon the pupal cuticle. The first and second pairs are hidden beneath the fore 

 wings, but on raising the latter (in pupae placed in spirit immediately after throwing off 

 the larval skin) the scars can be seeu, and tliey may even be detected in living puj)9e with 

 exceptionally transparent wings (e. g. Misella oxyaccmthce). The third and fourth pairs 

 of claspers nearly always leave conspicuous scars upon the fifth and sixth abdominal 

 segments (see Plate XXI. fig. 17 &c.). 



2. The Caudal Horn of Sjihinc/idce 8fG. — This structure always leaves a scar on the 

 pupa, even in those species in which it is feebly developed at the close of larval life (e. g. 

 Chcerocampa elpenor &c.). The scar is especially distinct in Macroglossa stellatarum. 

 In most pupae of Sphingidse there; is a well-marked depression on the eighth abdominal 

 segment behind the scar left by the horn. This must be due to the bending downwards 

 of the horn, which becomes quite horizonal before the larval skin is thrown off, so that 

 the posterior edge of its base and the continuous adjacent larval cuticle are depressed 

 and leave a permanent impress upon the yielding surface of the pupa. In Smerinthus 

 tillof the general surface of the pupa is corrugated, but the scar of the hoi"n is quite 

 smooth. (See Plate XX. figs. 13 & 11 for the scar in this species, fig. 2 for an unusually 

 prominent trace in ^S*. ocellatus, fig. 1 for the normal scar, figs. 9 and 10 for the scar in 

 S. popull.) 



The blunt horn of Undr07?us versicolor also leaves a large scar, very different in 

 appearance from the rest of the pupal surface (see Plate XXI. fig. 11 for an unusually 

 conspicuous example). 



Not only does a firm chitinous structure leave a distinct trace on the pupa, but soft 

 elevations of the surface, like those seen upon the first and eighth abdominal segments 

 of the larva of Acronycta psi, can be plainly detected upon the corresponding segments 

 of the pupa (see woodcut 1, x 2, in which the pupa is seen from its dorsal aspect). 



3. Other Larval Strtictures tohich can be detected on the Pupa. — The peculiar rough 

 plate upon the dorsal surface of the anal flap of the larva of Smerinthus tllice is repre- 

 sented by the extremely rough dorsal surface of the terminal spine of the pupa, and is thus 

 a valual)le aid to the identification of these two structures. In the larva of Pygcera 

 hucephala there is a " glabrous corneous black plate occupying the (dorsal surface of the) 

 anal flap " (Newman, ' British Moths,' 1869, p. 220). This is represented on the anterior 



