626 SUMMARY OF CURRENT RESEARCHES RELATING TO 



stimuli and transmits them to the perceptive centres by the optic nerves ; 

 and (3) a 'pigmentary apparatus, which absorbs the unutilised vibrations, 

 intercepts oblique rays, and isolates one ommatidium from another. 



But this analysis is incomplete as regards the eyes of some rapidly 

 flying insects, such as dragon-flies. For in the eyes of JEschna, for 

 instance, the author has proved the presence of an accommodation- 

 apparatus, which adapts vision to different distances. This apparatus 

 lies between the ommatidia, and consists of (a) elastic and extensile 

 elements (trachea?) ; and (b) contractile elements, genuine myofibrils. 



Wasps of Spain.* — Jose M. Dusmet y Alonso gives a systematic 

 descriptive account of the Vespidse, Eumenidaa, and Masaridze, of Spain. 



Development of Bees' Eggs. t— Otto Dickel has studied the forma- 

 tion of the blastoderm, the origin of the yolk-cells, the history of the 

 yolk-cells with especial reference to the blastopore, the formation of 

 the endoderm and mesoderm, and so forth. 



No distinction can be drawn between yolk-cells and the endoderm 

 established by invagination. Both are derived from the same material. 

 The endoderm of insects may be formed by yolk-cells, or by invaginated 

 material, or by both. The distribution of yolk-cells and invaginated 

 material may be such, that in the earlier stages the yolk-cells form the 

 endoderm of the median region, and the invaginated material forms 

 the polar regions. Between endoderm and mesoderm there are the 

 closest relations, for both arise in intimate interdependence. 



Parthenogenesis in Telenomus.t— J. TV. Wasschew has established 

 the occurrence of "arrenotokie" (male-producing) parthenogenesis in the 

 genus Telenomus, a member of the family Proctotrupidae among 

 Ichneumonids. 



Formica exsecta as Hill-builders in Swamps.§ — N. Holmgren 

 has noted a number of special features in the hills of this species, which 

 are the result of competition with the mosses (Sphagnum and Poly- 

 trichum) in the vicinity. Here the hills are more numerous and smaller 

 than in drier regions, and there is an absence of large roads leading to 

 them. Being drier than the surrounding parts, they are soon invaded 

 by Polytricham strictum, which gradually forms a thick carpet covering 

 and penetrating the heap. The form of the hill becomes altered as 

 building is carried on on the side remote from the Polytrickum attack ; 

 in other words, they withdraw from it. In course of time the habitable 

 part becomes too small for the ants, and migration occurs. This 

 explains the large number of small ant-heaps in the swamps. Ulti- 

 mately the ants are beaten and the moss remains in possession — for a 

 time. But before the hill is completely covered by the Polytrichum, 

 Sphagnum presses in upon the latter and in general crushes it out. It 

 will thus be seen that the ants play a not unimportant role in the 

 swamps, forming layers upon which the mosses can grow. 



* Mem. Soc. EspaS. Hist. Nat., ii. (1903) pp. 119-225. 



t Zeitschr. wiss. Zool., Ixxvii. (1904) pp. 481-527 (2 pis. and 46 figa.). 



t Zool. Anzeig., xxvii. (1904) pp. 578-9. 



§ Zool. Jahrb. xx. (1904) pp. 353-70. 



