180 AN INTRODUCTION TO ENTOMOLOGY 



Deviation from the usual type of incomplete metamorphosis. — The 

 more strikinj:^ deviations from the usual type of hemimetabolous devel- 

 opment are the following: 



The Odonata. — In theOdonata the wings of the naiads are inverted; 

 these insects resembling in this respect the Saltitorial Orthoptera. 

 What is the upper surface of the wings with naiads becomes the lower 

 surface in the adults, the change taking place at the last ecdysis. 



The Ephemerida. — In the Ephemerida, there exists the remarkable 

 phenomenon of an ecdysis taking place after the insect has left the 

 water and acquired functional wings. The winged instar that is 

 interpolated between the last aquatic one and the adult is termed the 

 sub-imago. 



g. COMPLETE METAMORPHOSIS 



{Holometaboljis* Development) 



The representatives of several orders of insects leave the egg-shell 

 in an entirely different form from that they assume when they reach 

 maturity; familiar examples of these are caterpillars which develop 

 into butterflies, maggots which develop into flies, and grubs which 

 develop into beetles. These insects and others that when they 

 emerge from the egg-shell bear almost no resemblance in form to the 

 adult are said to undergo a complete metamorphosis or a holometdbolous 

 development. 



The Holometabola. — Those orders that are characterized by a 

 holometabolous development are grouped together as the Holometab- 

 ola. This group includes the Neuroptera, Mecoptera, Trichoptera, 

 Lepidoptera, Diptera, Siphonaptera, Coleoptera, and Hymenoptera. 



This grouping together of these orders, while convenient for dis- 

 cussions of metamorphosis, is doubtless artificial. It is not at all 

 probable that the Holometabola is a monophylitic group. In other 

 words complete metamorphosis doubtless arose several times inde- 

 pendently in the evolution of insects. 



The term larva. — The form in which a holometabolous insect 

 leaves the egg is called larva. The term was suggested by a behef of 

 the ancients that the form of the perfect insect was masked, the Latin 

 word larva meaning a mask. 



Formerly the term larva was applied to the immattu"e stages of all 

 insects; but more recent writers restrict its use to the immature in- 



*Holometabolous: holos {Ji><'Oi), complete; metahole {tiera^oX-o), change. 



