344 MANUAL OF ZOOLOGY. 



sects live parasitically upon the bodies of Birds or Mammals, 

 or upon other Insects. 



DISTRIBUTION OF INSECTS IN TIME. The most ancient 

 remains of Insects at present known to us are from the De- 

 vonian rocks of North America. Here occur several forms 

 apparently belonging to the Neuroptera (or Pseudo-neiiropterd}, 

 In the Carboniferous rocks the remains of Insects are more 

 abundant, and we find examples of several orders (such as the 

 Coleoptera, Orthoptera, and Neuroptera]. The orders Hymen- 

 optera and Lepidoptera are not certainly known to occur till the 

 Secondary period is reached; and in the Tertiary rocks we 

 find representatives of almost all the existing orders. Amber, 

 which is a fossil resin, has long been known to contain many 

 insects in its interior (in certain specimens) ; and all of these 

 appear to belong to extinct species, though amber, geologically 

 speaking, is not an ancient product. 



CHAPTER XXXIX. 



DIVISIONS OF INSECTA. 



THE class Insecta includes such an enormous number of 

 species, genera, and families, that it would be impossible to 

 treat of these satisfactorily otherwise than in a treatise espe- 

 cially devoted to entomology. Here it will be sufficient to give 

 simply the differential characters of the different orders, draw- 

 ing attention occasionally to any of the more important points 

 in connection with any given family. 



As already said, the Insecta are divided into three divisions, 

 termed Ametabola, Hemimetabola, and Holometabola, according 

 as they attain the adult condition without passing through a 

 metamorphosis, or have an incomplete or complete metamor- 

 phosis. The Insects which come under the first head (viz., 

 Ametabola) are not furnished with wings in the adult condition, 

 and the four orders which compose this section are commonly 

 grouped together under the name Aptera. By some, however, 

 this division is entirely rejected, and the orders in question 

 are placed amongst the Hemimetabola, or even grouped with 

 the Myriapoda. Indeed, it is certain that the orders of the 

 so-called Apterous Insects are not, strictly speaking, scientific 

 divisions. It is, however, a matter of convenience to retain 

 them in a separate form, as it is by no means absolutely 



