REVIEWS. 191 



of the subject, from Pausanias, Livy, Pliny, &c., to the present 

 time. In some few instances lie tells us a species may be mi- 

 gratory in one district, and in another may be truly sedentary, e.g., 

 Acridium Americanum and possibly Pachytylus cinerascens. Mr. 

 Thomas also assures us that there are but two European species 

 of true migratory locusts {Pachytylus migratorius and Caloptenus 

 italicus). 



The following important result is eventually arrived at : — 

 " Our conclusion, therefore, is that the migratory habit or instinct 

 of locusts, whether in the Eastern or Western Continent, is 

 directly attributable to the arid condition of the area in which 

 they originate ; that the unusually dry and rarefied state of the 

 atmosphere is the chief factor in originating this instinct ; that 

 while it affects, to a greater or less extent, all parts of the insect, 

 its chief influence is produced by its effect on the tracheae and 

 air-sacs. It also must have a tendency to harden the in- 

 teguments, and to shrink or lessen the size of the softer parts. As 

 a very general rule, more species of the (Edipoid groups are 

 found in barren areas than of the Acrididse ; the Acrididie, on the 

 other hand, are generally fleshier, or, to use a botanical term, 

 more succulent than the Qj]dipod8e ; it follows, therefore, that the 

 effect of unusual dryness would manifest itself much sooner in 

 the Acridians than in the ffidipodeans, and such is undoubtedly 

 the fact." Then follows an examination of the varying eflect of 

 the dry years 1874—6 on the "trim" Caloptenus hivitattus, the 

 " more lubberly " C. differ entialis, and the " wild, ferocious- 

 looking" C. spretus. The fallacy of the periodicity theory of 

 migratory flights is well shown, although in both Continents there 

 appears to be a tendency to their occurrence about every eleven 

 years. 



The report is full of valuable meteorological and agricultural 

 data bearing on the various phases of the locust question. It 

 contains some very exhaustive chapters on the anatomy and 

 histology of the Orthoptera, by Mr. Packard and Mr. Minot, 

 admirably illustrated with fifteen plates and several woodcuts. 

 Mr. Riley contributes further facts on the natural enemies of the 

 locusts ; the interesting, and previously but slightl}' known, 

 economy of the Bomhylildce being illustrated with a coloured 

 plate. Mr. Scudder contributes a list of the Orthoptera collected 

 by Dr. Packard in the Western States, twenty of the new species 

 described being figured. This volume also contains a large 



