many cases, no doubt, this is accompanied by changes in the legs, and in the internal organization. An originally mandibulate mouth, however, like that of a beetle, could not, I think, have been directly modified into a suctorial organ like that of a butterfly or a gnat, because the intermediate stages would necessarily be injurious. Neither, on the other hand, for the same reasons, could the mouth of the Hemiptera be modified into a mandibulate type like that of the Coleoptera. But in _Campodea_ and the _Collembola_ we have a type of animal closely resembling certain larvæ which occur both in the mandibulate and suctorial series of insects, possessing a mouth neither distinctly mandibulate nor distinctly suctorial, but constituted on a peculiar type, capable of modification in either direction by gradual change, without loss of utility. In discussing this subject, it is necessary also to take into consideration the nature and origin of wings. Whence are they derived? why are there normally two pairs? and why are they attached to the meso-and meta-thorax? These questions are as difficult as they are interesting. It has been suggested, and I think with justice, that the wings of insects originally served for aquatic and respiratory purposes. In the larva of _Chloëon_ (Pl. IV., Fig. 1), for instance, which in other respects so singularly resembles _Campodea_ (Pl. III., Fig. 5), several of the segments are provided with foliaceous expansions which serve as respiratory organs. These so-called branchiæ are in constant agitation, and the muscles which move them in several points resemble those of true wings. It is true that in _Chloëon_ the vibration of the branchiæ is scarcely, if at all, utilized for the purpose of locomotion; the branchiæ are, in fact, placed too far back to act efficiently. The situation of these branchiæ differs in different groups; indeed, it seems probable that originally there were a pair on each segment. In such a case, those branchiæ situated near the centre of the body, neither too much in front nor too far back, would serve the most efficiently as propellers: the same causes which determined the position of the legs would also affect the wings. Thus a division of labour would be effected; the branchiæ on the thorax would be devoted to locomotion; those on the abdomen to respiration. This would tend to increase the development of the thoracic segments, already somewhat enlarged, in order to receive the muscles of the legs.