GEOLOGICAL AGE. 
9 
Geological Age. 
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 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 
all other land animals of the world taken together, and a recent 
computation 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 
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 
Other Features. 
