Metamorphosis 



637 



entiation among different species of anurans 

 as reported by these workers suggests that 

 httle confidence can be placed in the cyto- 

 logical correlations that have been adduced 

 in specific instances. Unpublished studies 

 by the writer of the growth and histology 

 of the anterior pituitary during the develop- 

 ment of the tadpole in R. pipiens likewise 

 failed to reveal any change in growth rate 

 or histological picture indicative of a marked 

 change in activity related to metamorphosis. 

 It may be concluded that, although the 

 activation of the thyroid at the beginning of 

 metamorphosis is clearly dependent upon a 

 thyrotrophic hormone from the pituitary, it 

 cannot be stated whether the initiation of 

 thyroid activity results from an increase in 

 thyrotrophic hormone production or from 

 other factors. 



CONTROLLING FACTORS IN INSECT 

 METAMORPHOSIS 



The metamorphosis of hemimetabolous in- 

 sects is commonly described as taking place 

 in stepwise fashion, a small step with each 

 larval* molt. In holometabolous forms, on 

 the other hand, transformation from larva 

 to adult is said to take place in one stage, 

 the pupal stage, at the end of larval develop- 

 ment (Fig. 220). However, it must be real- 

 ized that in all insects there is a process of 

 remodelling taking place at all stages. In 

 hemimetabolous insects noticeable progress 

 toward the adult condition is made with 

 each molt. Nevertheless the last molt, some- 

 times called the metamorphic molt, usually 

 presents a considerably greater amount of 

 change than any of the earlier ones. In that 

 respect this last molt is similar to the pupal 

 stage of holometabola. Also, although the 

 holometabolous animal does not seem to 

 change toward the adult condition in any 

 obvious way during larval molts, yet the 

 imaginal discs may undergo considerable 

 microscopic change. It is to be expected, 

 therefore, that the basic mechanisms of meta- 

 morphosis would be similar in both types of 

 insects. 



The earlier experimental work on insect 

 metamorphosis gave only small promise of 

 showing a common basic pattern throughout 

 insects, since many contradictory conclusions 

 were drawn from work on different species. 



* The term "larva" will be here used for the 

 young of hemimetabolous insects (nymphs) as well 

 as for those of holometabola in order to emphasize 

 the equivalence, as revealed by experimental analy- 

 sis, of the early instars in both types of insects. 



However, in recent years largely through 

 the comprehensive analyses of Wigglesworth 

 on the bug Rhodnius (Hemiptera, hemimetab- 

 ola) and Williams on the silkworm 

 Platysamia (Lepidoptera, holometabola) a 

 clarification and simplification of our under- 

 standing of the metamorphic process in in- 

 sects has taken place. It might be best for the 

 sake of clarity to present in outline the 

 interpretation of these experiments and then 

 to proceed to consider the extent to which 

 the work on other forms can be fitted into 



MGd.N.S.C. 

 Lat.N.S.C. 



Fig. 221. Relations of head endocrine glands in 

 an insect, generalized type. The brain contains two 

 groups of neurosecretory cells on each side (Med. 

 N.S.C. and Lat. N.S.C.). These are connected by 

 nerves to the corpora cardiaca (C.C.) and corpora 

 allata (C.A.). (After Cazal, '48.) 



the pattern seen in Rhodnius and Platy- 

 samia. 



Before doing so, however, it may be well 

 to describe briefly the basic morphology of 

 the organs with which we shall be concerned. 

 Each side of the brain of the insect commonly 

 possesses two groups of large neurons, a 

 medial and a lateral group (Fig. 221). These 

 neurons contain stainable droplets which ap- 

 pear to be secretions. Presumably these se- 

 cretions are released as hormones, either 

 directly into the blood stream or first being 

 conducted down the axons as in other neuro- 

 secretory cells (Cazal, '48; Scharrer, '52a; 

 Thomsen, '52). 



Lying more posteriorly in the head, on 

 either side of the anterior end of the aorta, 

 are two pairs of glands. The more anterior 

 pair of these are the corpora cardiaca (C. 

 C.) and behind them the corpora allata 

 (C.A.) (Fig. 221). These are joined to the 

 brain and to each other by nerves. In some 



