124 ESSENTIALS OF ZOOLOGY 



Insects exhibit a great deal of variation from the structures 

 illustrated by the study of such a type as the cockroach, but 

 the essential features are not departed from. The modifications 

 mainly affect the mouth parts, the wings, and the proportional 

 development of the regions of the body and the stages of 

 development. While a perfect system of classification is still 

 a desideratum, the following orders, based on the condition 

 of the wings, may be recognised. Thus, obvious modifications 

 allow of classifying the two-winged insects or flies as Diptera, 

 the scale-winged butterflies and moths as Lepidoptera, the 

 membrane-winged bees, wasps and ants as Hymenoptera, the 

 sheath-winged beetles as Coleoptera, the straight-winged 

 cockroach, earwig, leaf insect, locust, cricket, and grasshopper 

 as Orthoptera. Some members of the groups mentioned are 

 wingless, but other characters serve to show their relationship, 

 and there are groups which are characteristically without 

 wings. Such are assumed to fall into Apterygota, which 

 never have been winged, and Anapterygota, which have 

 secondarily become wingless. But the assumption is open to 

 question, and the classification is artificial and imperfect. 



The wingless groups have a simple development, the young 

 being hatched with a form similar to that of the adult. The 

 Orthoptera and one or two other groups differ only in that 

 wings are produced. The large majority pass through the 

 stages already mentioned, and the successive phases of life 

 are sharply marked by morphological peculiarities. 



Insects are economically important in various ways. 

 Several species are useful, as the bee, the cochineal insect, and 

 silk moth ; many serve a useful purpose in the f ertilisation of 

 flowers ; and certain others, as the hymenopterous Ichneumons 

 and the dipterous hover flies, the Syrphidae, destroy harmful 

 insects. 



Insects and Diseases. But it is in their relationship to 

 disease that insects have in recent years been found to play an 

 important part. Before that period they were known as para- 

 sites, from which man was not exempt, and they may under 

 circumstances be developed in such numbers as to be a danger 

 to crops in the field and in the nursery, and to produce in 

 granaries and warehouses. 



