46 



stages of the insects included in those orders characterised by their 

 species having an incomplete metamorphosis, and the term " nymph " 

 to the active pupal stage preceding the appearance of the imago. 

 Many others, however, employ the term " nymph " to include all the 

 early stages between the egg and the imago. The "nymph" in this 

 sense, then, when applied to such insects as grasshoppers, &x., 

 denotes the young which lead an active life up to the time of the final 

 development of the perfect insect, that quit the egg in an advanced 

 condition of development, having regard to their powers of locomo- 

 tion, and that have at this time the mouth parts formed after the same 

 type of construction as those of the adult insect. We have, then, so 

 far seen that there are two distinct conditions of metamorphosis in 

 insects — that represented by the Orthoptera and that represented 

 by the Lepidoptera. 



In the Synaptera (i.e. insects without wings), however, development 

 is direct, the young differing neither in form, structure, nor habits 

 from the adult. Hence such insects are said to be ametabolous ; and 

 since these insects show no tendency whatever to undergo any trans- 

 formation, it is quite clear that metamorphosis is a phenomenon that 

 has been induced in insects since the first winged forms appeared, 

 and our knowledge of its details suggests that it has been intensified 

 with the specialisation of the most highly developed orders of the 

 class. As we have already said, the transformations of winged insects 

 vary in degree, and they fall naturally into the two groups — the 

 Heterometabola (with incomplete metamorphosis) and the Holome- 

 tabola (with complete metamorphosis). In the former we have 

 observed that the adults differ from the freshly hatched young chiefly 

 in having wings ; they have no inactive, resting, or pupal stage, and 

 the wings are only acquired after successive moults. We have also 

 seen that the orders belonging to this group comprise the Orthoptera, 

 Dermaptera, Platyptera, Ephemeridse, Odonata, Thysanoptera, and 

 Hemiptera. To illustrate this mode of metamorphosis any common 

 grasshopper will serve our purpose. Many of these have five moults, 

 and six stages or instars, five of which belong to the nymph stage. 

 In the first two stages there are no rudiments of the wings ; these 

 appear after the second moult. This development of rudimentary 

 wings is accompanied by slight changes in colour and size, and 

 similar changes occur at each moult, until, in the adult, the full 

 mature colouring and completely formed wings are developed. 



It is, however, in the phenomena presented hy the metamorphosis 

 of the Holometabolic section of insects that most interest centres. 

 In these the larva undergoes a definite number of moults or exuvia- 

 tions of the skin, generally constant for the same species, although 

 female larvae frequently have an instar more than those of the males. 

 After each moult there is generally a considerable change, and hence 

 in many larvae each stadium (or period between two moults) is 

 characterised by a particular form of armature or plumage. 



The term "larva," then, is specially applicable to the young of the 



