vi PERIPATUS, INSECTA 241 



III. The INSECTA represent the highest stage of arthropod evolution. 

 In fact as regards the high degree of evolution both of physical structure 

 and of psychical development, which finds its expression in intelligence 

 and in complexity of social organization, certain of the insects rank with 

 certain of the vertebrates as the most highly evolved animals at present 

 existing. The most nearly primitive members of the group are classed 

 together as the Aptera, which have not yet developed wings, and which 

 appear to possess, in some cases, vestiges 1 of appendages upon the 

 segments of the abdomen. 



The ORTHOPTERA include a large variety of well-known insects as 

 may be gathered from the table on p. 209. They constitute one of the 

 less highly evolved orders of insects : their mouth appendages are of the 

 comparatively undifferentiated type seen in the Cockroach (p. 227) : 

 they do not undergo metamorphosis, the change in form from young to 

 adult taking place in a series of small steps corresponding with the 

 ecdyses. Some of them, such as Cockroaches (" Black Beetles "), Ear- 

 wigs, and the migratory grasshoppers commonly called " Locusts," are 

 of economic importance from the damage they do. Others, such as 

 many of the true Locusts, Praying Insects, and Stick- and Leaf-insects, 

 are of special scientific interest from the beautiful protective resemblance 

 which they show in form and colour to such natural objects as leaves 

 or sticks which form their normal background. 



The HEMIPTERA, or Rhynchota, are easily recognizable by the fact 

 that the labium forms a long, usually jointed, proboscis which when not 

 in use is folded back beneath the thorax. As in the blood-sucking 

 Diptera the labium is traversed on its anterior side by a deep groove 

 which serves to contain and protect the sharp piercing stylets formed 

 by the mandibles and the first maxillae. The group includes two sections 

 which differ in the character of the wings. In the subdivision Homoptera 

 the two pairs of wings are as a rule alike. They include the Cicadas, 

 perhaps the noisiest of insects, in which in the males a special area of 

 exoskeleton on each side of the hinder part of the thorax can be thrown into 

 rapid vibration by muscular contraction so as to produce a characteristic 

 sound, in some species like the whistle of a railway engine. The Green- 

 flies or Aphides, including Phylloxera, a North American aphid which 



1 It is of advantage to be precise in the manner of using the two words 

 rudiment and vestige. The two terms agree in that each means a small-sized 

 representative of an organ. The difference is that the first is used for an 

 organ on the up-grade of evolution, the second for one on the down-grade. 

 Thus the representative of the wing in the embryo of a bird is a rudiment : 

 the reduced remnant of the wing in a running bird, such as a Kiwi, is a 

 vestige. 



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