ENTOMOLOGICAL SOCIETY OF ONTARIO. 91 



A not infrequent question is " Wiiat is an Insect "? " and for the benefit of many who 

 have not opportunity to study entomology, yet to whom some knowledge of the subject is 

 important, it may be answered by the author's brief and clear definition of the class Insecta ; 

 or Insecta Hexapoda. 



" Insects are small animals, having the body divided into three regions placed in 

 longitudinal succession, head, thorax and abdomen : they take in air by means of trachffe, 

 a system of tubes distributed throughout the body, and opening externally by means of 

 orifices placed at the sides of the body. They have six legs and a pair of antennje ; these 

 latter are placed on the head, while the legs are attached to the thorax, or second of the 

 three great body divisions ; the abdomen has no true legs, but not infrequently has ter- 

 minal appendages and, on the under surface, protuberances which serve as feet. Very 

 frequently there are two pairs of wings, sometimes only one pair, in other cases none ; 

 the wings are always placed on the thorax. Insects are transversely segmented — that is 

 to say, the body has the form of a succession of rings ; but this condition is in many 

 <;ases obscure ; the number of these rings rarely, if ever, exceeds thirteen in addition to 

 the head and to a terminal piece that sometimes exists. Insects usually change much in 

 appearance in the course of their growth, the annulose or ringed condition being most 

 evident in the early part of the individual's life. The legs are usually elongate and ap- 

 parently jointed, but in the immature condition may be altogether absent, or very 

 short ; in the latter case the jointing is obscure. The number of jointed legs is 

 always six." 



The amplification of this definition and the expositioa of the external and internal 

 structure, and of the functions of the various organs, occupy two chapters. Referring 

 to Parthenogenesis, or " the production of young without the concurrence of the male," 

 which sometimes occurs, the remarkable fact is noted that in a few species of saw-flies, 

 gall-flies and scale-insects no male is known, so that they must be considered as perpetually 

 parthenogenetic. The next chapter gives a valuable summary of the embryology and 

 metamorphoses. While the vast majority of insects are oviparous, the eggs deposited 

 varying greatly in number, size and shape, a few species bring forth living young, as in 

 the Aphididje (green-fly or plant-lice), which thus multiply with extraordinary rapidity, 

 A brief chapter follows on the classification, and ic can readily be understood that diversity 

 of opinion has existed, and may long continue, as to the most satisfactory arrangement of 

 the vast hosts of insects. As some 250,000 species have alrealy been described, and 

 several times that number undoubtedly exist, any scheme of classification must, under 

 oar present knowledge, fail to adequately provide for the reception of every form. Dr. 

 Sharp points out that owing to the present limited knowledge of the earlier stages of 

 insects, the only complete system of classification yet possible must be based upon the 

 structure of the adult forms. It is noted with pleasure that he does not consider it 

 necessary to make so many orders or primary divisions as has been the tendency of recent 

 authors. Instead of twenty, as recently proposed by Packard, he limits them, much to 

 the advantage of the ordinary student, to nine, viz., Aptera, Orthoptera, Neuroptera, 

 Hymenoptera, Ooleoptera, Lepidoptera, Diptera, Thysanoptera, and Hemiptera. 



The Aptera are designated as " small insects, with weak outer skin, destitute 

 throughout life of wings or their rudiments, but with three pairs of legs ; antennie large 

 or moderate in size." It is pointed out however that this definition does not clearly 

 difi"erentiate them from mainy of the young individuals of other orders, and that the order 

 does not, as its name might indicate, include all wingless insects. Two suborders are 

 present : Thysanura, with the abdomen composed of ten segments, and OoUem- 

 bola, of not more than six. The study of thes3 insects is attended with more than 

 ordinary difficulty, as their habits and fragile structure make them troublesome to collect 

 and preserve. Campodea, supposed by many authors to represent one of the most 

 primitive types of insect, and therefore of unusual interest, is said to be " so extremaly 

 delicate that it is ditficult to pick it up, even with a camel's hair brush, without breaking 

 it." The CoUembolaare the " Spring-tails," two of the three families having the abdomen 

 provided with a leaping apparatus which enables them to jump about in a very vigorous 



