The Microscope and Biology. By W. Wesche'. 423 



sexual dimorphism is commonly found, the females are very diffi- 

 cult to distinguish, and I have been able to separate female insects 

 by a comparison of this part. 



When I say " inexplicably " I do so advisedly, as in what manner 

 the various setae, points, tubercles, folds and differences in shape 

 can influence any particular species is at present an enigma ; varia- 

 tions in genitalia are a check on hybridism, but how do these 

 modifications further that end ? 



IV. — The Genitalia. 



I have treated this subject at some length in a former paper,* 

 to' which I refer those who wish to pursue this difficult subject. 



The study of these organs is of great use in the separation of 

 species. The microscopist who has seen that the genitalia are 

 identical, is not confused by the variability in colours, however 

 remarkable, or deceived by the similarity of appearance so per- 

 plexing in the Sarcophagidse or the Lucilidce, and in so many 

 Lepidoptera. 



Summary. 



I have several times mentioned facts that may be very ancient 

 history to the entomologist, but are useful to the microscopist of 

 average experience. But besides this, I have collected a number 

 of observations that I submit are, at all events, out of the beaten 

 track, and I shall enumerate these in the order in which they 

 occur in the paper. 



1. Modifications in the general structure that are guides as to 

 the habits of insects are discussed. Figures of various types of 

 limbs are given in the plates. 



2. A number of the comparatively rare secondary sexual 

 characters in the female are given, confined however, to the Order 

 of Diptera. 



3. General rules are formulated for finding the nature of the 

 food from the characters of the mouth-parts, also confined to the 

 same Order. Figures of the various types to illustrate the rules 

 are given in the plates. 



4. A number of observations of the food of various mandibu- 

 late insects belonging to other Orders are given, and illustrated by 

 photography in the plates. 



5. The appearance of digested food in various insects is dis- 

 cussed, and suggests that an identical process of digestion occurs 

 in all Orders, and in all habits (predaceous or otherwise). 



* " The Genitalia of Both Sexes of Diptera." Trans. Linn. Soc. Ser. 2 (Zool.) 

 ix., Part 10, July 1906. 



