THE PHYLA ARTHROPODA AND ONYCHOPHORA 



follows. Normally, the presence of a royal pair inhibits the development 

 of additional reproductive individuals, except in parts of the nest distant 

 from the royal chamber. This inhibition appears to depend on the produc- 

 tion by the king and queen of specific substances v^hich all members obtain by 

 ingesting the secretions or feces of the royal pair. If a colony is deprived of its 

 king or its queen, supplemental reproductive individuals of the appropriate sex 

 w^ill develop within a short time, from among the undifferentiated nymphs. If 

 an "orphaned" colony is separated from a normal one by wire screens which 

 prevent all contact and all transfer of secretions, the isolated group will 

 develop reproductive individuals in the normal manner. If only a single 

 screen separates the groups, however, preventing transfer of secretions but 

 allowing the members of the two colonies to touch each other with their 

 antennae, the orphaned colony will develop supplementary reproductive 

 individuals but kill them as fast as they are produced. Thus it is evident 

 that caste development is regulated by two sets of stimuli, one sensory, the 

 other chemical; it also appears that the two media of information transfer 

 operate at different levels. Similar experimentation indicates that the same 

 kinds of cues regulate colonial organization among other social insects such 

 as bees, wasps, and ants. 



In termites, as in ants, wings are developed by reproductive individuals in 

 preparation for a seasonal swarming period. After the establishment of a new 

 colony, the wings are discarded. 



The mouth parts of termites are mandibulate, and the life cycle is pauro- 

 metabolous, with gradual metamorphosis. 



Order Odonata (toothed) — dragonflies and damsel flies. These are aqua- 

 tic during the nymphal or naiad stages but give rise to aerial adults; the 

 life cycle is thus hemimetabolous (Fig. 15.27). There are two pairs of 

 membranous wings in the adult, and the mouth parts of all stages are 

 mandibulate. 



Adult dragonflies are wonderfully efficient fliers, skimming and hovering 

 gracefully over the surface of ponds and streams. Although superstitiously 

 considered dangerous to man, they are in fact beneficial, destroying in- 

 numerable small flies and mosquitoes which they capture in flight. The 



Fig. 15.26. Thysanura. A typical thysanuran, the firebrat, Thermobia domestica. (Redrawn, 

 after Illinois Natural History Survey, from H. H. Ross, A Textbook of Entomology, second edi- 

 tion, copyright 1956 by John Wiley and Sons, Inc., printed by permission.) 



459 



