Posted on Tuesday 30 June 2009
In proportion as their reasonable suspicions increased, the termiteswould carefully avoid all doubtful looking mantises; but, at the sametime, they would only succeed in making the mantises which survivedtheir inquisition grow more and more closely to resemble the termitepattern in all particulars. For any mantis which happened to come alittle nearer the white ants in hue or shape would thereby be enabled tomake a more secure meal upon his unfortunate victims; and so the veryvigilance which the ants exerted against his vile deception would itselfreact in time against their own kind, by leaving only the most ruthlessand indistinguishable of their foes to become the parents of futuregenerations of mantises.
Once more, the beetles and flies of Central America must have learned byexperience to get out of the way of the nimble Central American lizardswith great agility, cunning, and alertness. But green lizards are lesseasy to notice beforehand than brown or red ones; and so the lizards oftropical countries are almost always bright green, with complementaryshades of yellow, grey, and purple, just to fit them in with the foliagethey lurk among. Everybody who has ever hunted the green tree-toads onthe leaves of waterside plants on the Riviera must know how difficult itis to discriminate these brilliant leaf-coloured creatures from thealmost identical background on which they rest. Now, just in proportionas the beetles and flies grow still more cautious, even the greenlizards themselves fail to pick up a satisfactory livelihood; and so atlast we get that most remarkable Nicaraguan form, decked all round withleaf-like expansions, and looking so like the foliage on which it reststhat no beetle on earth can possibly detect it. The more cunning you getyour detectives, the more cunning do the thieves become to outwit them.
Look, again, at the curious life-history of the flies which dwell asunbidden guests or social parasites in the nests and hives of wildhoney-bees. These burglarious flies are belted and bearded in the veryself-same pattern as the bumble-bees themselves; but their larvae liveupon the young grubs of the hive, and repay the unconscious hospitalityof the busy workers by devouring the future hope of their unwillinghosts. Obviously, any fly which entered a bee-hive could only escapedetection and extermination at the hands (or stings) of its outragedinhabitants, provided it so far resembled the real householders as to bemistaken at a first glance by the invaded community for one of its ownnumerous members. Thus any fly which showed the slightest superficialresemblance to a bee might at first be enabled to rob honey for a timewith comparative impunity, and to lay its eggs among the cells of thehelpless larvae. But when once the vile attempt was fairly discovered,the burglars could only escape fatal detection from generation togeneration just in proportion as they more and more closely approximatedto the shape and colour of the bees themselves. For, as Mr. Belt haswell pointed out, while the mimicking species would become naturallymore numerous from age to age, the senses of the mimicked species wouldgrow sharper and sharper by constant practice in detecting and punishingthe unwelcome intruders.