638 



Metamorphosis 



insects the C.A. of the two sides are fused to 

 form an unpaired median gland. In the 

 thorax of the insect strands of large cells are 

 found. These are commonly designated as 

 "prothoracic glands." 



In the larval and early adult dipteran the 

 arrangement of some of these structures is 

 different (Fig. 222). The cells equivalent 



Rinc| Gland 



Aorta 



{ ^ , , .e^ Esophac^us 



Fig. 222. Head endocrine glands in a larval dipteran, 

 Eristalis tenax (after Cazal, '48). 



to the C.A. are located dorsal to the aorta, 

 those corresponding to the C.C. are ventral 

 and joined to the C.A. region by lateral 

 lobes presumably equivalent to the protho- 

 racic glands (Possompes, '50a). The entire 

 complex forms the so-called ring gland of 

 Weismarm. 



RHODNIUS 



Rhodnius is a blood-sucking bug which 

 molts at a definite interval (varying from 

 15 to 28 days depending on the stage) after 

 a single meal of blood. Wigglesworth ('34- 

 '51) in a series of investigations found evi- 

 dence that the molt is brought on by a 

 hormonal stimulus from the brain. For ex- 

 ample, he found that decapitation before a 

 certain stage (the critical period) after the 

 meal prevents the subsequent molt, but 

 decapitation later does not. Brains from 

 post-critical-period animals induce molt in 

 non-activated abdomens. When active ani- 

 mals or parts thereof are joined in parabiosis 

 to non-activated tissues the stimulus to molt- 

 ing is transmitted to the latter, even when 

 the parts are joined by capillary tubing (Fig. 

 223). By transplantation the active region 

 of the brain was found to be the part of the 

 protocerebrum containing the large neuro- 

 secretory cells. 



The brain hormone in Rhodnius, acting 

 alone, induces a molt in which larval organs 

 undergo complete transformation to the adult 



condition. However, when the corpora allata 

 are active they change the effect of the brain 

 hormone, with the result that the larval 

 tissues lay down structures appropriate for 

 the next nymphal stage, not for the adult 

 condition. During all larval instars except 

 the last in the normal development of the ani- 

 mal both brain and C.A. are active. The molt 

 which ensues is therefore a larval molt from 

 which the animal emerges as the next larval 

 instar. At the last larval stage the C.A. fails 

 to be active in secreting its hormone. There- 

 fore under the unmodified influence of the 

 brain hormone the larval structures undergo 

 complete transformation to the adult con- 

 dition. 



The evidence upon which this interpreta- 

 tion of C.A. function is based is varied and 

 complex. Much of it derives from the re- 

 markable way in which whole insects or 

 body regions from different insects can be 

 joined together in parabiosis (Fig. 223). By 

 joining insect parts, activated and inacti- 

 vated, with and without brain and corpora 

 allata, the effectiveness of the stimulus in 

 each such part can be tested. In addition to 

 this technique, the brain and corpora allata 

 can be dissected out of larvae and adults 

 and implanted into larvae of various stages. 

 Also, by cutting the head immediately be- 

 hind the brain, preparations can be secured 

 which have C.A. intact but no brain. The 

 evidence derived from these varied proced- 

 ures shows that an insect body-part molting 

 under the influence of fifth (last) instar 

 body fluids assumes the adult condition ir- 

 respective of its original instar level. Im- 

 planted C.A. from younger instars than the 

 fifth change the molt so that nymphal struc- 

 tures are produced. Conversely, fifth instar 

 animals metamorphosing under the influence 

 of stimuli derived parabiotically from earlier 

 instars do not form adults but instead form 

 giant sixth instar nymphs. Curiously, even 

 adults which, of course, do not normally molt 

 at all, can be made to molt when in para- 

 biosis with active nymphal instars. When 

 the metamorphic stimulus comes from an 

 early instar the molt may lead to at least 

 partial reversion to nymphal characters. 

 Wigglesworth ('48) found that the fifth in- 

 star larva was more, rather than less, effec- 

 tive in inducing complete transformation in 

 its parabiont when its C.A. was intact than 

 when it was removed. This rather unex- 

 pected result was interpreted as showing that 

 not only does the C.A. not secrete any of its 

 hormone in the fifth instar, but it absorbs 

 or otherwise counteracts any such hormone 



