INTEODUCTIOK 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 {Collcmhola, or " Spring-tails," 

 and Thysamira, or " Fish-insects ") have no wings, the Diopter a (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 

 metamorpliosis, 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 

 {Le-pidoijUra, Diptera, Hymenoptera, Ncitroptera, and Coleoptcra) in which 

 the larva is cylindrical and worm-like, and the pupa, or chrysalis, 

 perfectly quiescent and helpless ; while in the others (Hemiptera and 

 Orthopterct) 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 apparent development, as the successive casts of skin 

 or moultings are gone through. 



While the Orders of Insects were named by Linn^us, 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 (inandibles) ; (3°) a pair of accessory masticatory jaws 

 (maxillce); and (4°) an under lip (labiicm), which is formed by the 

 more or less complete coalescence of a second pair of maxillee. 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 (Phryganeidm). 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 



