178 



AN INTRODUCTION TO ENTOMOLOGY 



with a sexual generation. And within the series of parthenogenetic 

 forms there may be an alternation of winged and wingless forms. In 

 some cases the reproductive cycle is an exceedingly complicated one ; 

 and different parts of it occur on different food plants. 



The Thysanoptera. — In the Thysanoptera, as in most other insects 

 with a gradual metamorphosis, the nymphs resemble the adults in the 

 form of the body, and the wings are developed externally; but the last 

 nymphal instar is quiescent or nearly so and takes no nourishment. 

 This instar is commonly described as the pupa. 



/. INCOMPLETE METAMORPHOSIS 



(Hemimetabolous* Development) 



In three of the orders of insects, the Plecoptera, Ephemerida, and 

 Odonata, there exists a type of metamorphosis in which the changes 



Pig. 20I. — Transformation of a May-fly, Ephemera varia; A, 

 adult; B, naiad (After Needham). 



that take place in the form of the body are greater than in gradual 

 metamorphosis but much less marked than in complete metamorpho- 

 sis. For this reason the terms incomplete metamorpJiosis and hemi- 

 metabolous development have been applied to it. 



Both incomplete metamorphosis and complete metamorphosis are 

 characterized by the fact that the immature instars exhibit adaptive 

 modifications of form and structure, fitting them for a very different 

 mode of life than that followed by the adult. This is often expressed 

 by the statement that the immature instars are "sidewise developed" ; 

 for it is believed that in these cases the development of the individual 

 does not repeat the history of the race to which the individual belongs. 



*Hemimetabolous: hemi (^mO. l^alf ; tnetabole (/iera/SoXi)), change. 



