ARTHROPODA 



19 



Fig. 7. — Head of 

 the bee, showing 

 compound eyes, the 

 three ocelH, and the 

 antennae. — Magni- 

 fied (after Orton, by 

 Dodge; Copyright, 

 1894, by Harper & 

 Brothers) . 



a weapon of defense provided with poison glands. From its nature the 

 sting is essentially onh' possessed by the females. 



Some insects on leaving the egg develop directly to the adult stage, 

 the larva in most cases differing from the adult prin- 

 cipalh' in the absence of wings. In such cases there 

 is a slight change of form with successive molts, 

 the wings being ultimateh' acquired. Here the meta- 

 morphic process is not thorough, and is therefore 

 referred to as mcomplete metamorphosis. The ma- 

 jorit}^ of insects when hatched from the egg bear 

 no resemblance to the adult, and there is no observ- 

 able gradual approach to this form. The larva is 

 characteristically worm-like and an active and vora- 

 cious feeder, a number of molts occurring with the 

 increase in size during this stage. There then 

 intervenes between the larval and adult stages a 

 period of pupation, during which the animal is quies- 

 cent and a series of changes 



occur in the body. At the 



conclusion of these changes the pupal case 



splits and the imago emerges, which, with 



the unfolding of the ap- 

 pendages and hardening 



of the cuticle, has in all 



essentials developed into 



the complete sexual 



adult. In this form 



of development the 



changes are distinct, and 



the process is referred 



to as complete meta- 

 morphosis (Fig. 8). 

 The duration of life 



in insects, including the 



stages of the egg, larva, 



pupa, and adult, usually 



does not extend bej^ond 



a year. With quite a 



number it is much 



shorter than this, while 



with others it maj' be a 



matter of several years, 



an extreme example of 



larval longevity being 



Fig. 9. — Diagram of termi- 

 nal segments of arthropod leg, 

 with muscles, a, articulation; 

 f, flexors; e, extensors Cafter 

 Boas, by Kirkaldy & Pollard). 



Fig. 8. — Metamor- 

 phosis of the House Fly, 

 showing oval, larval, 

 pupal, and adult stages. 

 On the right is an en- 

 largement of the foot; 

 on the left, the foot pad, 

 showing sticky, glandu- 

 lar hairs; on upper left, 

 a tsetse fly (from photo- 

 graph of drawing by 

 author). 



