GEOLOGICAL AGE. 9 



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, 

 aiicl 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 blow-fly 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 

 ' divisions of the Arthropoda, with the exception of the Proto- 

 tracheata (Peripatus), and possibly the Centipedes, 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 satis- 

 factorily 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 con- 

 clusion 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 fairly 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 

 Other Features. 11,1 i 



all other land animals ot the world taken together, and a recent 



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

 Lord Walsingharn, 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 

 are in advance of all other groups. In brightness of colour, beauty of pattern, 

 and gracefulness of form some of the species can hardly be equalled even by the most 

 gorgeous birds, while in mechanical perfection of structure, as testified by activity 

 and strength, others of the group are unsurpassed in the animal kingdom. It has 

 been stated that if a man could leap in proportion to his stature as far as a flea can 

 hop, he could clear at a bound a wall over one hundred feet high, and if he could sing 

 as loudly as the cicada, his voice could be heard for a distance of many miles. 

 Indeed, even in matters about which man is wont to especially pride himself, 

 such as those touching social organisation, he might with advantage go to the ant 



