20 



KNOWLEDGE. 



[January 1, 1901. 



insect which was described by Leach in 1815 as Machilis 

 inaritiina. These Bristle-tails may be found in numbers 

 around the rocky parts of our coasts, sometimes lurking 

 under stones, sometimes disporting themselves in the 

 sunshine on the smooth sides of a rock-pool. They 

 can run with a swift, gliding motion over the sui'face 

 of the rock, and ai-e able to leap nimbly when disturbed. 

 Without reckoning the long feelers and tail bristles, 

 M. maritima measures about 12 mm. {}^ inch) in length 

 {see Fig. 1). 



The structure of Bristle-tails is of gi-eat interest to 

 naturalists, since these insects have retained a number 

 of primitive chai-acters. They remain wingless through- 

 out life, and they are especially remarkable in having 

 paired limbs on sevei'al of the hind-body (abdominal) 

 segments. It is believed, therefore, that they represent 

 to some extent the far-off, many-legged, ancestors of our 

 present-day six-legged insects. In Machilis these 

 abdominal limbs (iig. 1, II.-X.) are more numerous 

 than in any other genus of the Thysanm-a, being present 

 on all the segments, from the second to the tenth 

 inclusive. Those bonie on the tenth segment ai-e the 

 long-, jointed, bristle-like organs, known as cercopods, 

 which ai-e to be seen also in some winged insects — 

 mayflies and stoneflies, for example. 



But a point in the structure of the Machilis suggests 

 that though these abdominal limbs may tell us of 

 ancestral forms with many jsairs of legs, yet they do 

 not correspond exactly with the six legs of the thorax. 

 The haunch in the second and third pairs of legs in 

 Machilis canies, in addition to the functional legs, a 

 small pointed haiiy appendage (Fig. 1, 2-3), which agrees 

 closely in form and size with the abdominal limbs. It 

 seems, likely, therefore, that two-branched limbs occurred 

 on all the segments in the ancestral foiin, and that the 

 inner branch persists as the functional limb of the 

 thorax in insects generally, while the outer is preserved 

 in the abdominal appendages of the Bristle-tails. 



Close to these limbs we find small bladder-like sacs,, 

 which can be withdrawn (Fig. 1, h) or pushed cut 

 (Fig. 1, B), at the will of the insect. There is a pair on 

 each of the first, sixth, and seventh abdominal segments, 

 and two jjairs on each segment from the second to the 

 fifth inclusive. J. T. Oudemans,|| to whose researches 

 we owe most of our knowledge about Machilis mariUma, 

 gave much attention to the structui'e and probable 

 use of these organs, and came, somewhat doubtfully, to 

 the conclusion that they have a respiratory function. 

 When thrust out, they contain blood, and they might 

 well be used to absorb air in damp situations. But 

 Machilis breathes also, like the higher insects, by means 

 of air-tubes. These open to the exterior by paired air- 

 holes (stigmata) on the two hindmost thoracic segments 

 and the second to the eighth abdominal segments in- 

 clusive. The branching systems of tubes leading from 

 the various air-holes remain separate; they do not, like 

 those of the winged insects, unite to form continuous 

 ti-unks and networks. 



Like some other Bristle-taUs — the well-known " Silver 

 Fish insect " (Lepisma) of our houses, for example — 

 Machilis is covered with scales, resembling those found 

 on the wings of moths and gnats. The scales of Machilis 

 (Fig. 3) are of large size, about .1 mm. in length, and 

 are stiffened with thickened rods running lengthwise; 

 between these, shoi-t ti'ansverse markings can be dis- 

 tinguished arranged like the rungs of a ladder. The 

 scaly covering of Machilis maritima gives to the insect 



11 " Beitrage zur Keimtniss del' Tlivsanura mid Collcmbola." Bijd. 

 tot tie Dierkunde (Amstertlam), XVl'., 1888, pp. 147-227, pis. I. -III. 



during life a dark mottled aspect, which often agrees 

 closely with the appearance of the rocks among which it 



Fig. 1. — .l/a(7ii7i> iHaciYwna, Leach. Female seen from the Ventral 

 Aspect. M', M-, first and second pairs of maxilUv; 2, 3, appendages 

 on haunches of second and third legs; II.— X., limbs on abdominal 

 senmeuts; O, ovipositor; B. pi-otnuled, and b, withdrawn bladder-like 

 sacs on abdominal segments. Magnified 5 times. 



Fio. 2.— Mandibles (Mn.), and Maxillnla; (Mxl.) of Hansen 

 ("paraglossae" of most authors) dissected out of the head. Magnified 

 20 times. 



Fio. 3. — A Scale from the Back. Magnified 25 times. 



