174 AN INTROD UCTION TO ENTOMOLOGY 



It appears to be necessary that the original coxa be not removed in 

 order that the reproduction may take place. Figure 193 represents 

 a spider in our collection in which two legs, the left fore leg and the 

 right hind leg, were being reproduced when the specimen was captured. 



d. DEVELOPMENT WITHOUT METAMORPHOSIS 



{Ametabolous* Development) 



While most insects undergo remarkable changes in form during 

 their postembryonic development, there are some in which this is 

 not the case. In these the young insect just hatched from the egg is 

 of practically the same form as the adult insect. These insects grow 

 larger and may undergo slight changes in form of the body and its 

 appendages; but these changes are not sufficiently marked to merit 

 being termed a metamorphosis. This type of development is known 

 technically as ametabolous development. 



Development without metamorphosis is characteristic of the two 

 orders Thysanura and Collembola, which in other respects, also, are 

 the most generalized of insects. 



The nature of the changes in form undergone by an insect with an ametabolous 

 development is illustrated by the development of Machilis allernata, one of the 

 Thysanura. The first instar of this insect, according to Heymons ('07), lacks 

 the clothing of scales, the styli on the thoracic legs, and the lateral rows of eversi- 

 ble sacs on the abdominal segments; and the antennae and cerci are relatively 

 shorter and consist of a much smaller number of segments than those of the adult. 

 These changes, however, are comparable with those undergone by many animals 

 in the course of their development that are not regarded as having a metamorpho- 

 sis. In common usage in works on Entomology the term metamorphosis is used 

 to indicate those marked changes that take place in the appearance of an insect 

 that are correlated with the development of wings. 



In addition to the Thysanura and the Collembola there are certain 

 insects that develop without metamorphosis, as the Mallophaga 

 and the Pediculidas. But their ametabolous condition is believed to be 

 an acquired one. In other words, it is believed that the bird-lice and 

 the true lice are descendants of winged insects whose form of body and 

 mode of development have been modified as a result of parasitic life. 



The Ametabola. — Those insects that develop without meta- 

 morphosis are sometimes referred to as the Ametabola. This term was 

 first proposed by Leach (1815), who included under it the lice as well 

 as the Thysanura and Collembola. But with our present knowledge, if 

 it is used it should be restricted to the Thysanura and Collembola, 

 those insects in which a development without metamorphosis is a 

 primitive not an acquired condition. 



*Ametabolous: Greek a, without; metahole (nera^oX-^) , change. 



