52 BRITISH LEPIDOPTKRA. 



the larva is clinging as is its wont, the weight of all the parts of 

 the body anterior to the third abdominal segment is only indirectly 

 supported by means of the claspers. He further points out that 

 the young larvae of all species which exhibit this habit, habitually 

 rest on the underside of leaves, and, therefore, have the dorsal area 

 pointing downwards. Under these circumstances "the organism reacts 

 upon the strain, and the muscular body-walls strongly contract upon 

 their fluid contents in such a manner as to produce compensating 

 rigidity, and thus give to the body the curve which is characteristic of 

 the attitude. The Sphinx-like attitude is to be explained as the com- 

 bined effect of gravity and of muscular reaction upon the anterior un- 

 supported parts of the body. The muscular arrangements, which are 

 most favourable for counteracting these strains, are also made use of 

 in the older larvae for the maintenance of a feebly marked Sphinx-like 

 attitude, when the larva is seated on the upper side of a horizontal 

 twig. The attitude is most strongly marked when the larva is resting 

 on a vertical twig, because gravity tends to draw the anterior part of 

 the body backwards as well as downwards. These large larvae 

 habitually rest on vertical twigs, with the head uppermost, because the 

 twig itself is approached from its base, and gradually stripped of 

 leaves towards its apex. The essential dependence of the attitude upon 

 gravity is well seen, when a vertical twig, with a larva upon it, is 

 carefully bent downwards, so that the strain is in the opposite direction, 

 and tends to bend the anterior part forwards instead of backwards. 

 Under these circumstances the larva begins to yield to the strain in a 

 few minutes {Trans. Ent. Soc. Land., 1888, p. 675). 



An interesting subject of enquiry is the evolution of the Geometrid 

 form. The fact that this form is found, in a more or less modified 

 condition, in certain Noctuid larvae, has suggested an alliance between 

 the two groups. It seems very probable, however, that this similarity 

 has been brought about by somewhat similar needs, the Geometrid 

 form being, in many respects, a very specialised one. Many Noctuid 

 larvae that have the full number of prolegs when adult, are more or 

 less Geometrid in form when young. It appears probable that this 

 form has been developed in order to give these larvae a greater reach 

 (1) to obtain their food, (2) to travel from one twig to another. The 

 Geometrids are essentially herbaceous and arboreal in their habits, 

 remaining on their food-plants the whole of the day, so also are the 

 Plusias and other Geometriform Noctuids. The Noctuids that have a 

 Geometrid form of progression when young, also, at this period of 

 their lives, remain on their food-plants, but when they gain the 

 hitherto absent prolegs, they climb down the plants and hide at the 

 roots, or under the ground by day, ascending the plant again to feed 

 by night. The comparatively low-feeding Geometrid larvae are, 

 as a rule, small species, and the bushy herbs on which they 

 feed, bear to their power of reach much the same proportion as 

 the larger trees bear to the reaching power of the larger larvae. 

 Another view of the matter suggests itself, viz., the necessity of 

 Geometrid larvae to travel more quickly than other tree-feeding larvae. 

 The Sphingids, Saturniids, Lasiocampids, Dicranurids, Catocalids, etc., 

 are specially protected by spines, hairs, etc. The Geometrid larva is 

 naked, usually only protected by the resemblance of its colour to its 

 environment, and by its power to remain rigid and motionless, When 



