CLASSIFICATION OF INSECTS. 295 



derived from the metamorphosis, by which the class may be divided into insects with a perfect 

 metamorphosis, and those with an imperfect metamorphosis or no metamorphosis at all. For many 

 years the former of these methods of division was the one adopted by almost all naturalists, and 

 it has certain advantages in its favour, especially the practical one that, being founded exclusively 

 upon the characters presented by the insects in the perfect state, the student has no occasion to 

 trouble himself about the transformations which they have undergone in order to decide their 

 place in the system. This advantage, however, is more apparent than real, for except in the case of 

 a single so-called order of mandibulate insects the character of the metamorphosis forms part of that 

 of the order; and, on the other hand, there are almost always ample structural distinctions by which 

 the members of the orders can be separated, even without reference to their transformations. 

 Further, taking into consideration the points that have been raised since the resuscitation of the 

 doctrine of the evolution of organic forms by the works of Mr. Darwin and others, it must be admitted 

 that from this point of view the nature of the metamorphosis is of great importance ; and for 

 these reasons we adopt a primary division of the class of insects in accordance therewith. 



The only difficulty that presents itself more strongly from this point of view than from that of 

 the structure of the mouth is how we are to deal with certain small groups of insects which undergo 

 no metamorphosis at all. These creatures, which are generally of small size and low organisation, 

 may be residues of groups formerly more numerous and abundant, in which case they ought probably 

 to be kept distinct from the other existing orders of insects ; or especially in the case of the parasitic 

 forms, which are the most numerous, they may be degraded representatives of the orders to which 

 they appear to be most nearly related. We shall adopt both these views for the different types of 

 insects with no transformations, and arrange the orders as follows : 



I. INSECTS WITH A PERFECT METAMORPHOSIS. 



A. With biting mouths, the mandibles always distinct : 



1. Fore wings horny or leathery, forming a pair of sheaths (elytra] covering the 



abdomen and hind wings, and generally meeting in a straight line down the 



middle . ........ 1. COLEOPTERA. 



2. All the wings membranous : 



a. Veins in the wings few ; prothorax united with the mesothorax . . 2. HYMENOPTERA. 



b. A r eins in the wings numerous ; prothorax free . . . .3. NEUROPTEKA. 



B. With sucking mouths : 



1. Wings four, scaly ; maxillae forming a spiral proboscis . . . .4. LEPIDOPTERA. 



2. Wings not more than two : 



a. Two wings ; halteres ; thoracic segments united ; proboscis formed of the 



labium, enclosing bristles ...... 5. DIPTERA. 



b. Wings none ; thoracic segments distinct ..... 6. APHANIPTEHA. 



II. INSECTS WITH AN IMPERFECT METAMORPHOSIS OR WITH NONE AT ALL. 



A. With sucking mouths ; rostrum composed of the jointed labium enclosing bristles . 7. RHYNCHOTA. 



B. With biting mouths, of which the parts are exposed ; no organs of locomotion at the 



extremity of the abdomen 8. ORTHOPTERA. 



C. With biting mouths, the parts of which are usually very delicate, and concealed within 



the cavity of the mouth ; no wings ; no metamorphosis . . . .9. THYSANVRA. 



By many entomologists the Aphaniptera, or Fleas, are united with the Diptera, or two-winged 

 Of the insects with no metamorphosis, we have retained the order Thysanura, the members of 

 which have sometimes been united with the Orthoptera ; but of the parasitic forms, the true Lice are 

 referred to the Rhynchota, and the Bird-lice (Mallophaga) to the Orthoptera. The Bee parasites, 

 forming the order Strepsiptera of many writers, are placed among the Coleoptera. Some years ago 

 Prof. Westwood founded a distinct order (Achrioptera) for a small insect parasitic on the Canadian 

 Beaver. This has also been shown to belong to Coleoptera. The order Diploglossata, proposed by 

 M. de Saussure, in 1879, for a small African insect resembling a Cockroach, but presenting a second 

 labium, may be referred to the Orthoptera. 



W. S. DALLAS. 



