38 



tions, until the following summer to that in which the original larvae 

 were hatched, when they revert to the usual procedure of pupa and 

 imago. On the other hand, the family Sarcophagcc (flesh-flies) often 

 produce living larvee, and in the Hippoboscidce (forest-flies and 

 allied species) both egg and larval stages are passed in the body of 

 the parent, the young being produced in the form of pupje. 



The pupae of Diptera are divided into two sharply defined groups 

 ■ — the Orthorraptha and the Cydorrhaptha — and the entire order 

 falls under one or the other of these groups. In the former the 

 pupa is free or enclosed in the larval skin, showing more or less 

 clearly legs, head, etc., and the imago escapes by means of a T-shaped 

 opening in the back of the larval skin. The pupa of the common 

 daddy-longlegs is a good example of this group. In the Cydor- 

 rhaptha^ on the other hand, the pupa is always enclosed in the 

 hardened larval skin, forming a smooth barrel-shaped object, and the 

 imago escapes by means of a circular orifice at the anterior end of 

 this puparium. This is usually effected by means of an air-sac 

 situated on the frons of the insect, which, when distended, forces off 

 the cap of the puparium. This air-sac — the ptilinum — is afterwards 

 retracted into the head, and is seldom noticed in the mature fly 

 except when caught just after emergence. Some Diptera, especially 

 the Musddm, undergo a most complete metamorphosis in the pupal 

 state, the larval body being reduced to an absolute creamy pulp from 

 which the imaginal body-parts are subsequently built up. This 

 very complete metamorphosis, together with the specialized arrange- 

 ment of nerve-ganglia in the perfect insect, has led systematists to 

 place the Diptera high up among the orders of insects. The 

 geological record tends to confirm this view, as Diptera do not appear 

 until the Mesozoic or Secondary period, and are not met with in 

 any numbers till the Cainozoic or Tertiary period. To this epoch, 

 however, a considerable quantity of fossil Diptera have been referred, 

 and it is at this period also that the Hymenoptera and Lepidoptera 

 begin to appear, all three orders being thus coincident with the 

 appearance of the earlier forms of flowering plants. 



There is another side to the study of Diptera to which I should 

 like to draw attention. I refer to what is comprehensively termed 

 " bionomics." The " habits " of flies offer a fascinating field for 

 research, though a difficult one, as it is necessary to be very careful 

 not to deduce too much from the facts observed in order to help 

 them to fit in with some plausible theory or other. In mimicry, for 

 instance, one gets material from the Diptera that tells both for and 

 against the theory. The life-history of an Eristalis fly compared 

 with that of a bee refutes a frequently suggested reason for the 

 resemblance between two imagines, namely, that similar conditions 

 of growth have brought about similar results. The VoluceUce are 

 remarkably similar to the bumble-bees in whose nests their larvae 

 live, but it does not appear that the bees notice them either more or 

 less for the resemblance \ and Vohicella inanis, whose larvae are found 



