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THE ANIMAL KINGDOM 



Figure 17.4. Endocrine glands of the cockroach. The upper group He above the 

 esophagus in the head. The lower gland is ventral in the prothorax, strung among the 

 muscle cells. (After Bodenstein.) 



In the insects several endocrine glands are known, and a little is 

 known of their interactions. The hormonal control of molting is now 

 well understood. A molt is initiated by the intercerebral gland (Fig. 

 17.4), a part of the brain. The neurosecretory cells in this gland pass 

 their hormones along the axons to their expanded tips in the corpus 

 cardiacum, just behind the brain. The release of a prothoracicotropic 

 hormone by this gland sets in motion an irreversible series of events. 



The hormone is carried in the hemocoel to the prothoracic glands, 

 a pair of ectodermal glands in the ventral part of the prothorax, which 

 it stimulates to liberate a molt and metamorphosis hormone (m & m 

 hormone). This hormone acts directly on the epidermis, causing it to 

 secrete molting fluid and to start a new exoskeleton. In addition, the 

 m & m hormone causes the epidermis to assume the adult morphology. 



In young insects, however, the m & m hormone does not act alone. 

 Another pair of glands, the corpora allata, closely associated with the 

 corpus cardiacum, secrete a juvenilizing hormone. This hormone does 

 not prevent a molt but it does prevent metamorphosis, thereby preserv- 

 ing the juvenile morphology. The corpora allata are very active in 

 young insects but they gradually become less active, and finally lose their 

 power to preserve immaturity. A hormone analogous to this, which 

 preserves youthfulness, is unknown in vertebrates. 



Interference with the molting honnones has produced interesting 

 results. Fukuda, working with the silkworm, removed the corpora allata 

 from young caterpillars (Fig. 17.5). They pupated on the next molt and 

 emerged later as miniature adults. They were mature functionally as 

 well as structurally, and even reproduced. The converse of this experi- 

 ment was done by Wigglesworth on a blood-sucking bug, Rhodnius. 



