INTRODUCTIOK 
3 
known either as veins or nervures. The possession of six thoracic 
legs only, and of four thoracic wings, are the unmistakable marks of 
Insects properly so called. The number and position of the legs are 
quite constant ; but the lowest groups {Colhmhola, or " Spring-tails," 
and Tliysanum, or Fish-insects ") have no wings, the Diptem (House- 
flies, Gnats, &c.) have the hind pair of wings undeveloped, and in all 
the winged Orders cases of wingless forms occur. 
Closely associated with the possession of wings — which is the 
exclusive privilege of the adult insect — is the more or less complete 
metamorphosis, or series of changes from one stage or state of develop- 
ment to another, undergone in the course of progress from the egg to 
the Imago, or perfect Insect. This is very striking in those Orders 
{Lepidoptera, Diptcra, Hijmenoptera, Neuroptera, and ColeopUra) in which 
the larva is cylindrical and worm-like, and the pupa, or chrysalis, 
perfectly quiescent and helpless ; while in the others {Hemiptera and 
OrtlwpUra) the close resemblance of the larva when hatched to the 
adult, with the result that there is no true pupal state, but unbroken 
activity throughout life, renders the acquisition of wings a matter of 
more gradual and apjDarent development, as the successive casts of skin 
or moultings are gone through. 
While the Orders of Insects were named by Linnaeus, and are still 
for the most part conveniently grouped, in accordance with characters 
peculiar to the wings, a more trustworthy basis for their classification, 
as far as external structure is concerned, is found in the parts of the 
mouth. When these are carefully studied, they are found in the several 
Orders to be fairly constant modifications of the typical insect-mouth, 
which consists of ( i °) a single horny upper lip (lahrum) articulated to 
the fore-part (clypeus) of the head; (2°) a pair of principal seizing or 
biting jaws {mandibles) ; (3°) a pair of accessory masticatory jaws 
(maxillce) ; and (4°) an under lip (lahmm), which is formed by the 
more or less complete coalescence of a second pair of maxillge. This 
typical form of mouth, so well adapted for seizing, holding, and tearing 
up food, is most fully shown in the Orthoptera (Grasshoppers, Praying- 
insects, Leaf-insects, Dragon-flies, &c.), and Coleoptera (Beetles). In 
the Neuroptera (Ant-lions, Lace-wing flies, Caddice-flies, &c.), while 
the masticatory type prevails, there exist certain groups in which 
there is modification towards a suctorial type {Panorpidce), or con- 
siderable atrophy of the mouth-organs generally {Phryganeidce), In the 
Hymenoptera (Ants, Bees, Wasps, Ichneumon-flies, &c.), there is a 
series of gradations from the masticatory to a combined masticatory 
and suctorial mouth. Lastly, the Orders Hemiptera (Bugs, Cicadas, 
Aphides, &c.), Diptera (House-flies, Gad-flies, Gnats, &c.), and Lepid- 
optera (Butterflies and Moths) are exclusively suctorial, the mouth- 
organs being profoundly modified to form a channel for liquid food. 
As compared with the other two suctorial Orders, in which all the 
typical mouth-parts are well expressed, though altered in order to the 
