Januaky 1, 1901.] 



KNOWLEDGE, 



21 



lives.' Similar protective colouring is to be seen in our 

 other British species — .1/. poli/poda (Linn.) — which is 

 often abundant in stony places in the hill-countries. 



For our knowledge of the habits of Jlachilis mari- 

 timi], as of its sta'ucture, we are chiefly indebted to 

 Oudemans. He observed the insects cai-efully on the 

 Dutch coast, where they frequent the stones of dykes and 

 searwalls, and the wooden piles of sluices. Fx'oni May 

 to November they are veiy lively, a.nd often wander 

 about singly. " It seems," he writes, " that they greatly 

 like warmth. I have seen them on a hot August day run 

 and leap very briskly on the hot stones of a dyke." 

 In winter they withdi'aw into holes and chinks, where 

 they crowd together in great numbei-s. Only full-grown 

 individuals are to be seen in autumn and winter, but 

 both adults and young in midsummer, whence it follows 

 that the young are hatched in spring, and that 

 the insects live for more than a year. "' They are," 

 writes Oudemans, " very inquisitive. If I approached 

 a place (except in winter time) where they were to be 

 found they usually came to the light. Did I approach 

 too near, they began to nin and jump about. If they 

 fall into the water it does not matter, since they arc 

 hardly wetted thereby. They can move over the 

 surface-film of the water, and even make .short leaps there- 

 on. Usually they reach the shore again after a short time, 

 or find an object up which they can climb. I have often 

 seen this." He goes on to describe how he kept specimens 

 in captivity on the siirface-film of water in a glass 

 vessel up whose smooth wall they could not make their 

 way. " They remained there forty-eight hours. When, 

 afterwards, I gave them a chance of regaining dry land, 

 they took the opportunity, and were perfectly right 

 again after a few moments." It seems, thei'cfore, that 

 Machilis is well enough adapted to a life by tlie sea^- 

 shore. They appear to feed on vegetable refuse, but, 

 like many creatures higher in life's scale, they fast 

 during winter. 



No one can examine Machilis without being struck 

 by its general likeness to some other, and more familiar, 

 creatures of the sea^shore — the s.and-hoppers and their 

 relations belonging to the crustacean order Amphipoda, 

 wliich have lately been so ably described in these 

 columns by Mr. Stebbing. Everyone knows that the 

 Crustacea fonn a class of animals distinct from the 

 Insecta, yet both cla-sses are included in the great Race 

 of joint«d-l egged animals or Arthropoda. Very divei'se 

 opinions have prevailed among zoologists as to the 

 relationship between insects and cmstaceans. It was 

 at one time thought that the Bristle-tails, the most 

 primitive of insects, as well as the cnistaceans, could be 

 traced back to an ancestral form resembling the well- 

 known zoi-a-larva of a crab.*" Of late years, however. 

 Ies3 importance has been attached to larvae as indicative 

 of relationships, and the view that there is but little 

 connection between insects and crustaceans has been 

 gaining ground.** But quite recently the problem has 

 been attacked from a fresh point of view, and a rather 

 close relationship between the Bristle-tails and the lower 

 malacostracous Crustacea — the Amphipods and Isopods 

 — has been advocated by the Swedish zoologist Hansen. ft 



It is well known that a typical insect, like a cock- 

 roach or a beetle, has three pairs of jaws — a pair of 



r K. iraeckel. " The History of Creation." Loudon, 1876. 



*• F. W. Futton and others "Arc the Arthropoda a Natural 

 Group? " yatural Science, Vol. X., 1S97, pp. 97-117. 



+t " A Contribution to the MorpholoiiT of the Limbs and Mouth 

 Partsof Crustaceans and Inseets " Ann'. Afar/. Nat. IlUt. (K) XII 

 1893, pp. 417-434. Hee also Nat. Set. {loe. cit.), pp. 103-5. 



mandibles and two pairs of maxUlae, the second of 

 which are more or less completely fused together to 

 form a " lower lip " (labium). In Crustaceans wc also 

 find a pair of mandibles and two pairs of maxilla;, but 

 in the Amphipods and Isopods the next pair of limb.s, 

 the first jaw-feet or maxillipeds, are also included in 

 the head region, and become partially joined to form a 

 " lower lip." Now Hansen has suggested that a pair of 

 structures foinid in the head of the Thysanura, united 

 basally with the tongue, especially well developed in 

 Machilis (Fig. 2, Mxl.), and situated between the 

 mandibles and maxilla, arc in reality a pair of jaws 

 which con-espond with the first maxilla; of cmstaceans. 

 If this view be correct, there are four pairs of jaws in 

 the Bristle-tails, and the con'cspondence of these insects 

 with the sand-hoppers, woodlice, and their relations be- 

 comes remarkably close, especially when we remember 

 that the number of segments behind the head is exactly 

 the same (thirteen) in both groups. Hansen states that 

 vestiges of these " maxillul:e " arc to be found in tlie 

 cockroach and other insects ; the Bri.stle-tails, therefore, 

 seem to have retained a pair of jaws which the higher 

 members of the class have almost or altogether lost. 

 The mandibles of the Thysanura, too (Fig. 2, Mn), have 

 many points of likeness to those of Crustacea. Another 

 coiTespondcnco with the sand-hoppers and woodlice is 

 brought out by Oudemans, who states that the cyo in 

 Machilis as in those crustaceans has a layer of hypo- 

 dermis-cells between the corneal facets and the crystal- 

 lino cones. 



As we watch MnchilU maritima disporting itself 

 around the rock-pools, the enquiry is suggested as to 

 the various coasts which it haunts. All around the 

 shores of our own islands — even to the remote Orkneys 

 and Shetlands, as well as the Irish Aran — it has been 

 found ; as also on the rocky co^asts of Nonvay, of 

 Holland, of France, and Spain, and even on the Canary 

 Isles lying out in the Atlantic off the shores of North 

 Africa. It does not seem to range to the eastern 

 European coasts. 



So far as we know, Machilis is incapable of crossing a 

 scarcha.nnel of any breadth. Its presence on these various 

 v/cstern continental a.nd island shores tends to show, 

 therefore, that at some former period there must have 

 been a continuous continental coast-line along which it 

 could migrate. This humble insect furnishes one of the 

 many distributional facts which point to the existence in 

 Tertiary times of continental land lying north and south 

 along the western margin of the present European area. 

 And its presence on the Canaries lends support to the 

 view- — once highly popular, then discredited, now be- 

 ginning again to find upholders — that this old western 

 continent may have stretched away far into the 

 Atlantic. 



The mere fact that our little Bristle-tail haunts these 

 rocky coasts calls up visions of a vanished continent, and 

 sh.jws how land and sea areas have changed in the 

 course of the earth's history. And when we study the 

 structure of the insect we are carried back to a past 

 still more remote. As we see a Machilis gliding over 

 some ancient Archaean rock, we imagine the primaeval 

 sea in which the fragments of that rock gathered as 

 sediment. In that same sea, maybe, lived the far-off 

 common ancestors of the insects and crustaceans of 

 to-day. No "record of the rocks " preserves the form of 

 these primitive arthropods for our study; but we can 

 guess at their nature by help of those characters which, 

 passed on through countless generations of living 

 creatures, still survive in our marine BrLstle-tail. 



