45 Studies in the Theory of Descent. 



very probable that in this case the form-relation- 

 ship of the caterpillar gives more correct informa- 

 tion as to the blood -relationship of the species 

 than that of the imago in any case the larvae 

 show a different form-relationship to the ima- 

 gines. 



Just as in the case of butterflies there are 

 many genera of Sphingidce which can be based on 

 the structure of the larvae, and which agree with 

 those founded on the imagines. 



Thus, the genus Macroglossa is characterized 

 by a straight anal horn, a spherical head, and by a 

 marking composed of longitudinal stripes, these 

 characters not occurring elsewhere in this com- 

 bination. The nearly allied genus Pterogon, on 

 the other hand, cannot be based on the larvae 

 only, since not only is the marking of the adult 

 larva very distinct in the different species, but the 

 anal horn is present in two species, whilst in a 

 third (P. CEnothera) it is replaced by a knob-like 

 eye-spot. The genus Sphinx (sensti strictiori) is 

 distinguished by the simple, curved caudal horn, 

 the smooth, egg-shaped head and smooth skin, and 

 by a marking mainly composed of seven oblique 

 stripes. The genus Deilephila is distinguished 

 from the preceding by a dorsal plate, situated on 

 the prothorax and interrupting the marking, as 

 well as by the pattern, which here consists of a 

 subdorsal line with ring-spots more or .less 

 numerous and developed ; the skin also is rough, 



