INVERTEBRATA, ORYPTOGAMIA, MICROSCOPY, ETC. 67 



In each family of tlie Diptera these organs manifest a single 

 structural plan ; while a gradual modification is undergone as we 

 pass from one family to another ; the highest rank helonging, not as 

 Dafour stated, to those forms showing the greatest numher of centres, 

 but to those in which concentration is carried to its utmost limit. 



Another conclusion which has been reached by this study is, that 

 the previous opinions as to conceiitration, during the process of develop- 

 ment, being a rule for the nervous centres in insects, must be 

 abandoned in the face of the fact that no less than five families of this 

 order exhibit a decentralization or disunion during the process of 

 development. 



The order may, according to these facts, be divided as follows : — 



I. Dijjtera in which some ganglia become united together in the 

 passage to the nymph stage : — Families, Tipulidae, &c. (embracing 

 the old group Nemocera). 



II. Diptera in which ganglia become disunited — some being trans- 

 ferred to the abdomen — during metamorphosis : — Tabanidse, Syrphidfe, 

 Conopidaj, Stratiomydae, the Acalypteran Muscidaa, Sepsinas, Platy- 

 stominte. 



III. Diptera in which the thoracic and abdominal ganglia remain 

 united during metamorphosis: — Muscidae calypteratte, OEstridae, 

 Hippoboscidae, Nycteribiidae. It should here be noted that the 

 embryo always has the ganglia distinct from each other. 



The above divisions are further broken up thus : — 



Group 1. Two or three thoracic, six abdominal centres : — XylophagidaD, 



&c., including all the Nemocera but those of the next two 



groups and of the seventh. 

 „ 2. Three thoracic, Jive abdominal : — Scenopinidae. 

 „ 3. Two thoracic, five abdominal : — Therevidae. 

 „ 4. One thoracic, five abdominal : — Tabanidae, StratiomydsB. 

 „ 5. One thoracic, two abdominal : — Syrphida3. 

 „ 6. One thoracic, one abdominal: — Conopidae, Acalypteran 



Muscidae. 

 „ 7. Two thoracic, no abdominal : — Dolichopodidae. 

 „ 8, A single thoracico-abdominal ganglion : — The families included 



in the thii-d of the three primary groups — Hippoboscid^, 



&c. 



Homologies of the Parts of the Labium in Orthoptera.* — 

 Altliough this organ has been described as unpaired, yet M. Chatiu 

 considers it conijnirable in its various parts to the jaws of the masti- 

 cating insects. M. Brulle failed when attemjjting a similar study on the 

 Coleoptera ; but it is in the Orthoptera that the solution is to be found. 

 Thus in Locusta vii-idit^sima, the large basal piece of the labium 

 represents the united second maxilla) ; an upper plate, the united first 

 pair ; a palpigcrous appendage is distinguishable with care from the 

 latter, carrying the palj)s ; a lamina is placed internally to the palps, 

 and carries two lobes, the exteruul of which is shown by duvcl(i])mcnt 

 and study of different genera to be the galea, the internal lobe being 



* ' Couiptca Ruiidu^j,' Ixxxix. (1879) i>. G52. 



F 2 



