THE INSECTS 2971 



sect development would suppose the two to belong to the same category of animals. 

 In these two orders, as well as in some others, the new-born young has the appear- 

 ance of a fleshy grub; and the grub-like condition is retained unchanged, except in 

 size, until the time for the last molt approaches. It then undergoes a startling 

 change of condition, and, losing its organs of sense and ceasing to feed, passes into 

 a state of quiescence, during which the final changes in its organization are more or 

 less rapidly passed through, and the final molt sets free the mature insect, perfect 

 in all its structural details. 



The immature stages of insects that present a complicated development of 

 this kind are variously spoken of as grubs, maggots, caterpillars, or more com- 

 prehensively, larvae; while the quiescent stage is termed the chrysalis or pupa, 

 and the final sexually mature stage the imago or perfect insect. Moreover, such 

 species are said to undergo a complete metamorphosis, or to be holometabolous, as 

 opposed to those like the cockroach, whose growth is accompanied by but little 

 change of form, and are said to present an incomplete metamorphosis or to be 

 ametabolous. It must not, however, be supposed that all insects are either com- 

 pletely or incompletely metamorphic in their development. The familiar types 

 that we have mentioned exhibit almost, although not quite, the extremes of change 

 that are offered in the class; but between these occur other types which show 

 developmental phenomena more or less intermediate in their nature, being less 

 complicated than those of the blowfly and more complicated than those of the 

 cockroach. An account of these various methods of development will be given 

 under each order as it is described. 



Like the Crustacea, Arachnida, Millipedes, and all the main 

 ge divisions of the Arthropoda, with the exception of the Prototracheata 

 {Peripatus}, and possibly the Cintipedes, Insects are an exceedingly ancient group, 

 having left their remains in strata of Silurian age. The exact nature and affinities 

 of these primeval remains has not, however, yet been satisfactorily determined, 

 and some authors indeed seem to doubt whether they are rightly referred to insects. 

 Still there is no question that species of this group flourished in abundance during 

 the Carboniferous period; but the conclusion that all the known fossil insects from 

 these strata form a natural order, distinct from all the existing groups of this rank 

 can hardly be regarded as finally established, seeing that, in the opinion of some 

 authors, they are assignable to places in our classification of existing species, and 

 are nearly related to the orders Orthoptera (cockroaches, grasshoppers, and dragon 

 flies) and Hemiptera (bugs and plant lice). In the Secondary rocks insect remains, 

 considering the small chances of the preservation of such creatures in stratified 

 deposits are fairl} r abundant; and none of the species present ordinal differences from 

 those which now exist. So, too, the hosts of species that have been discovered in 

 Tertiary deposits, in the amber beds and elsewhere, are referable to existing orders. 

 It has been estimated that in numbers of species insects excel all 

 BS other land animals of the world taken together, and a recent com- 

 putation has put the total of described forms at 250,000, and yet, according to 

 Lord Walsingham, only about ten per cent, of existing species have hitherto been 

 discovered. But this is not the only respect in which the animals of this class 



