26 ENTOMOLOGY 



specialization, as in the family Muscidae. Such larvae as those of 

 mosquitoes are comparatively primitive. 



The relationships of Diptera to other orders are not evident, but 

 the order is in some respects like Mecoptera. Diptera possibly came 

 from forms like Mecoptera, or both orders may have arisen from neu- 

 ropteroid ancestors. 



The fleas, Siphonaptera, are usually placed next to Diptera, being 

 regarded as degenerate flies. 



The preceding diagram (Fig. 33) is a graphic summary of the gen- 

 ealogy of some of the orders of insects. The central group (T) is the 

 hypothetical thysanuroid source of all insects, including Thysanura 

 themselves. Though Thysanura and Collembola show no traces of 

 wings, even in the embryo, it should be borne in mind that all the other 

 insects probably had winged ancestors and that it is more reasonable to 

 assume a single winged group as a starting point than to suppose that 

 wings originated independently in several different groups of insects. 



