Mr Lamb, Venational Abnormalities in the Dipiera 397 



The third class is the most interesting and is shown in Fig. 9, 

 a species of Agromyza, Fig. 10 Cyrtoneura stabulans (Muscidae), 

 Fig. 11 , Sapromyza dedecor (Sapromyzidae), and Fig. 12, Noterophila 

 glabra (Drosophihdae). Krober also figures two similar cases, in 

 Mydaea urbana and Policies lardaria, both in the Anthomyiidae. 

 The small triangle formed looks just as if the cross- vein in its 

 development had torn the fourth vein into its two constituent 

 parts M^ and Mg: this condition is often bilateral. 



A similar formation of a triangle, but taking the form of a 

 cross-vein between the two branches M^ and M2 beyond the discal 

 cell, is shown in Fig. 13, Platypeza modesta (Platypezidae), this is 

 accompanied in both wings by small hang veins. A noticeable 

 feature is that the teratologies very frequently occur in the neigh- 

 bourhood of the discal cell, that is to say, in that part of the wing 

 where one would expect stress phenomena to occur owing to the 

 presence of veins at right angles. 



The above examples constitute almost all those that the author 

 has met with, and they occur in single specimens of various species 

 spread over many famihes: in some of those species hundreds of 

 specimens have been seen. This remarkable degree of uniformity 

 breaks down in a species of ortalid, Ptilonota guttata (Fig. 14). Here 

 the teratological diathesis is extraordinary, taking as a rule the 

 form of hang-veins, which vary from dots near the second vein to 

 simple veinlets or complex forms as shown in the figure: these 

 hang-veins are also common in the discal cell itself. Out of the 

 40 specimens in the Cambridge collection no fewer than 15 exhibit 

 one at least of these abnormahties, many of them more than one. 

 Mr Edwards has kindly informed the author that in the British 

 Museum set, about 5 out of 21 show small defects of the nature of 

 hang-veins or dots, and Mr J. E. Collin tells him that 9 out of his 

 23 specimens exhibit the same phenomenon. The species also 

 shows much inconstancy in the acrostichal bristles; further, the 

 author figured in the E.M.M. 1911, p. 216 a unique case of true 

 Batesonian teratology occurring in the same species, the specimen 

 having on one of the antennae a small accessory third joint bearing 

 two extra aristas. The species seems to be endowed with extreme 

 natural instability, and it would be of interest to hear of similar 

 occurrences in other families of insects. 



