236 SUMMARY OF CURRENT RESEARCHES RELATING TO 



than the rest and ends in a main nerve-fibre ; of the other processes, 

 those which do not end by anastomosis, branch slightly and appear to 

 grow thin and terminate freely beneath or between the hypodermic 

 cells. Frequently the larger nerves beneath the hypodermis exhibit 

 a sphei'ical dilatation, formed by five projections shaped like the 

 segments of a melon and separated by deep grooves ; each of these 

 segments contains a fusiform nucleated mass of protoplasm. The 

 author compares the nervous plexus to that observed in Nematodes by 

 Biitschli and Villot. 



Relation of Devonian Insects to Later and Existing Types. *^ — 

 Mr. Scudder is led to think that (1) there is nothing in the structure 

 of these earliest known insects to interfere with the conclusion that 

 the general type of wing-structure has remained unaltered from the 

 earliest times. (2) These insects were Hexapods, and seem to have 

 preceded both Arachnids and Myriapods. (3) They were all allied to 

 or were Neuroptera — using that term in its widest sense, and none 

 exhibit any special orthopterous, hemipterous, or coleopterous charac- 

 teristics. (4) They nearly all exhibit affinity to the carboniferous 

 Palseodictyoptera ; but they often have a more complicated structure 

 than those insects, and have a distinct facies of their own. (5) They 

 were of great size, the span of wing averaging 107 mm. ; the wings 

 were purely membranous, and they were probably aquatic in early 

 life. (6) Some, such as PI at ephemera, may be regarded as aberrant 

 forms of existing families ; others, such as Gerephemera, had a number 

 of parallel veins, such as is found in no other known insect. (7) An 

 abundance of insect life at that epoch is spoken to by their remarkable 

 variety of structure. (8) Not only do the Devonian insects differ re- 

 markably from all other known types, but some of them appear to be 

 even more complicated than their nearest living allies. (9) We can- 

 not therefore say that we are any nearer the beginning of things in 

 the Devonian than in the carboniferous epoch, so far as unity or 

 simplicity of type is concerned. Finally, (10) " While there are some 

 forms which, to some degree, bear out expectations based on the 

 general derivative hypothesis of structural development, there are 

 quite as many which are altogether unexpected, and cannot be 

 explained by that theory, without involving suppositions for which no 

 facts can be at present adduced." The Devonian seem to have but 

 little in common with the Carboniferous insects. The author concludes, 

 however, with expressing his conviction that earlier comprehensive 

 types did exist and should be sought for. 



Head and Mouth Organs of Diptera.t — Putting together the 

 observations of the principal writers on these subjects, M. A. Menzbier 

 concludes that: — (1) The insect head consists most probably of six 

 segments, each developed from a pair of embryonic elements. (2) The 

 first segment forms the hinder part of the head and includes the 

 simple eyes. (3) The second segment forms the front part of the 



* Amer. Journ. Sci., xxii. (1881) pp. 111-7 ; summarized from the Anniversary 

 Memoirs of the Boston Soc. Nat. Hist. 1880. 



t Bull. Soc. Imp. Nat. Moscou (1880) pp. 8-70 (2 pis.). 



