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ABSTRACT
Two Israeli aglyphous colubrid snakes seem to mimic venomous viperid snakes. Spalerosophis diadema cliffordi resembles sympatric desert viperids, especially Vipera persica fieldi, in girth, coloration and defensive behaviour. Coluber ravergieri nummifer resembles the sympatric Vipera palaestinae in length, coloration and defensive behaviour. Direct evidence of functional mimicry is absent, except that man is frequently misled. Because several conventional conditions are fulfilled, these appear to be two cases of Batesian mimicry. Any of five hypotheses may enable the evolution of such mimicry systems in which the model may be lethal. Some individuals of both mimic species greatly increase the morphological resemblance to the models by behaviourally flattening and triangulating the head in the defensive display. This behaviour is distinct from that of flattening the head as part of a general flattening of the body, to increase the area shown an enemy or in basking. Parallely, cobras are mimicked by colubrids which erect the neck and spread a hood. Both this and the head-triangulation behaviours may have evolved from the generalized body-flattening response. This may have its origin in the thermoregulatory (basking) flattening common to snakes and lizards.