no 



SCIENCE-GOSSIP. 



most anterior part, a transparent flap, " the undu- 

 lating membrane," grows. When the animal is 

 stationary this is folded over in such a manner as 

 to form a pocket leading to the oesophagus. In 

 this position it forms a net to catch the Infusoria, 

 etc., which serve as food. When the animal 

 swims the undulating membrane is retracted and 



Fig. 27. — Cyclidium glaucoma ( X335). 

 v, Undulating membrane. 



cannot be seen. That part of the peristomial 

 groove which does not bear this membrane is 

 provided with a strongly developed row of cilia, 

 which serve to sweep the prey into the net. A 

 contractile vacuole is situated on the dorsal side 

 near the posterior end, which is perhaps connected 

 with the surrounding water by a fine canal. In 



the anterior half of the animal a round nucleus is 

 present, and, lying by it, a paranucleus. The anus 

 is ventral and subterminal in position. The body 

 cilia are long in proportion to the body. The 

 length of the animal is about 80 to 100 microns. 



This species occurs in pond and ditch water, 

 swims rapidly, rotating the while, and sometimes, 

 when one imagines that it will remain stationary 

 for some while, springs rapidly away. It was 

 described by Ehrenberg under the name Paramae- 

 ciuvi chrysalis, and transferred by Dujardin to the 

 genus in which it now stands. 



Cyclidium glaucoma Muller, resembles Pieuron etna- 

 chrysalis very closely in structure. The following 

 points of difference may be observed. It is much 

 smaller, being only 20 to 30 microns in length, the 

 body cilia are much larger in proportion to the 

 body, and supplemented by a long hair which 

 grows from the posterior end. The undulating 

 membrane is not so large, and does not arrange 

 itself so clearly into the form of a pocket. 



Its habits and habitat are similar to those of the 

 preceding species. 



{To be continued.) 



ORIGIN OF SPECIES IN INSECTS. 



By J. W. Tutt, F.E.S. 



(Continued from page 78J 



Specialization to Food-plant. 



A DIFFERENCE of habit which results in the 

 isolation of allied species by causing the 

 appearance of the imagines at separate times of the 

 year is, however, only one of a number of habits 

 which result in isolating closely allied species. 

 There is, first of all, the specialization of two allied 

 species to two different food-plants of restricted 

 range, or restricted to different geological forma- 

 tions. This not only tends to keep species very 

 local, by confining them to the ground on which 

 the food-plant grows, but it also isolates them 

 from each other, owing to the necessity of the 

 insects, confined to a local plant for pabulum, 

 giving up any wandering tendency they may have 

 originally possessed. Species that have been or 

 are isolated in this way are numerous in our fauna. 

 Thecla w-albutn is confined to elm (Ulmus), T. pruni 

 to blackthorn (Prunus), Cupido minima is confined 

 to Anthyllis, Lycacna arion to thyme (Thymus), 

 Netneobius lucina to Primula, and Nisoniades tages to 

 Lotus ; Sphinx pinastri is confined to Pinus, Macro- 

 glossa fuciformis to honeysuckle ; and M . bombytiformis 

 to scabious; Adscita globulariae is confined to 

 Cenlaurea, A. statices to Rumex, and A. geryon to 

 Helianthemum, Sanothripa undulana and Halias 

 chlorana are confined to Salix, Hylophila bicolorana 

 to oak, Leiocampa dictaea is confined to Salix and 



Populus, L. dictaeoides practically to Betula. Lophop- 

 teryx cuculla is confined to Acer, L. carmelita to Beiula, 

 Notodonta trepida is confined to Quercus, so also is 

 Drymonia chaonia ; Centra bifida affects Populus, 

 C. furcula prefers Salix, and C. bicuspis feeds on 

 Betula and Alnus. It is possible to show that about 

 a third of the British lepidopterous fauna is more 

 or less distinctly specialized with regard to some 

 particular food-plant, and is therefore more or less 

 isolated from its allies equally specialized with 

 regard to some other fopd-plant or food-plants. 



Isolation by Difference of Habitat. 



Other habits may result in the isolation of 

 closely allied species. The habit which, in 

 Britain, confines the polyphagous Clisiocampa 

 castrensis to our coast marshes, whilst the almost 

 equally polyphagous and closely allied C. neustria 

 is generally distributed in gardens and hedgerows, 

 results in their almost complete isolation. The 

 habit that confines Nudaria senex to marshy 

 meadows and N. mundana to the neighbourhood 

 of old walls ; the habit that confines Spilosoma 

 urticae to marshes and allows S. menthastri to 

 abound in gardens and hedgerows everywhere ; 

 the habit that makes Dasychira fasceiina a coast 

 and D. pudibunda a woodland species, are sufficient 

 to illustrate another means by which isolation 



