GYRINUS. 



[ 301 ] 



GYROSIGMA. 



cies of this genus, is very commonly seen in 

 groups performing its gyrations upon the 

 surface of pools or rivers, whence it has re- 

 ceived the popular name of whirligig. 



The body is ovate, or elHptic and de- 

 pressed, the elytra black and shining. The 

 anteiuise are short and retractile within a 

 cavity in front of the eyes ; the basal joint mi- 

 nute, the second large, globular, and furnished 

 externally with an ear-like joint fringed with 

 colourless, flattened, hair-like processes ; the 

 remaining seven joints form a clavate mass, 

 being very short and closely united, the first 

 commencing by a very narrow base or pe- 

 dicle. The eyes are di\'ided into two parts 

 by a transverse septum, the upper of which 

 serves for viewing objects in the air, the lat- 

 ter those in water ; bv some authors these 

 insects are described as possessing four di- 

 stinct eyes. The terminal segment of the 

 abdomen is furnished with two retractile 

 cihated lobes. The two fore-legs are long, 

 and of the ordinary form, whilst the four 

 hind legs (PI. 27 . fig. 5), which are used as 

 oars, are short, flat, and dilated ; the femur 



[d] and tibia (e) somewhat triangular, the tibia 

 also fringed with short spines and long, flat- 

 tened filaments ; in the middle pair of legs 



(e) the latter exist on both margins, whilst 

 in the hind legs these are present only on the 

 outer margin. The tarsi («) are five-jointed, 

 the three basal joints produced on the inside 

 into long, flat, leaf-like lobes fringed with 

 spines ; the fom'th joint is of about the same 

 size, and semicircular, the fifth being very 

 short and attached to the fourth near the 

 end, and both are fringed on their outer 

 margin with flattened filaments resembling 

 those upon the tibia ; all the tarsi are fur- 

 nished with two distinct claws. 



The anterior tarsi of the male differ 

 from those of the female, as in Dytiscus. 

 The circulating currents can be seen in the 

 hind legs. 



The larva (PI. 28. fig. 19), which is aquatic, 

 is of a dirty-white colour, long, narrow, and 

 depressed, resembling a small centipede ; it 

 consists of thirteen segments including the 

 head. Its antennae are filiform and four- 

 jointed ; the eyes numerous and tubercular, 

 grouped on each side of the head. The 

 three pairs of legs are attached to the eight 

 anterior segments of the body ; the remain- 

 ing segments are furnished on each side with 

 a branchial filament, excepting the last, 

 which has two of them, and four minute 

 conical points, bent downwards, and used by 

 the insect when in motion. 



BiBL. Westwood, Introduction, Sfc. i. 

 p. 105. 



GYROPUS, Nitzsch. — A genus of mandi- 

 bulate Anoplura (Insects), of the family 

 Liotheidse. 



Char. Tarsi two-jointed, with a single claw. 



Mandibles without teeth ; maxillary palpi 



conical and four-jointed ; labial palpi none ; 



antennae four-jointed; thorax two-jointed; 



abdomen ten-jointed. 



G. ovalis (PI. 28. fig. 8). Head ferrugi- 

 nous, transverse, with a lateral produced 

 lobe on each side ; thorax and legs ferrugi- 

 nous ; abdomen nearly orbicular, yellowish- 

 white ; claws long, cm*ved and strong ; length 

 1-48". 



Found upon the guinea-pig {Cavia co- 

 haya). 



G. gracilis. Head and thorax ferruginous; 

 abdomen elongate, segments with atransverse 

 striated band at each suture ; ungues very 

 short and minute ; length 1-36". 

 Found also upon the guinea-pig. 

 BiBL. Denny, Anoplur. Monographia. 

 GYROSIGMA, Hass. {Pleurosigma, Sm.). 

 — A genus of Diatomaceae. 



Char. Frustules single, free, longer than 

 broad; front view linear or linear-lanceolate; 

 valves navicular, sigmoid, with a longitudinal 

 line, and anodule in the centre and at each end. 

 The group of species arranged in this ge- 

 nus shoidd properly form a subgenus of 

 Navicula, inasmuch as the sigmoid form of 

 the valves, upon which the distinguishing 

 character is founded, does not exist in all 

 the species of Gyrosigma to a greater extent 

 than that in which it occurs in some species 

 of Navicula ; in some, its only indication is 

 a slight inequality in the two halves of the 

 valves. The median line and nodules consist 

 of an internal thickening of the valves at the 

 corresponding parts, the line is best seen in the 

 front view (Pi. 11. fig. 16) ; it is occasionally 

 found in a fractured valve, projecting as a solid 

 highly refractive rod, the thinner adjacent 

 portions of the valve being broken away; 

 for brevity, it may be called the keel. 



The valves exhibit spurious striae, arising 

 from the existence of rows of dots (depres- 

 sions), of which we have already treated 

 under DiATOMACEiE. These striae and dots 

 are in most species very difficult to detect, 

 requiring the use of oblique light, and the 

 stops; the principles which should guide in 

 the search for them have been explained 

 under Angular aperture ; the prelimi- 

 nary preparation of the valves is also essen- 

 tial (DlATCMACE^, p. 202). 



