The Evolution of Protective Displays in the Saturnioidea and Sphingidae (Lepidoptera)

1. The protective display behaviour of 35 species of Saturnioid and 6 species of Sphingid is described. The displays are classified into the following types: a. Generalised Escape Behaviour; b. Rhythmic Displays, composed of rhythmic movements of the wings, legs or abdomen, often highly co-ordinated, and repeated many times in succession; c. Sustained Static Displays of two types, in which static postures are maintained for several minutes' duration ; d. Mixed Displays, in which the insects maintain a Sustained Static Display at low response intensities, but give a rhythmic performance when the intensity of display is high ; e. Vestigial Displays, which may relate to any of the foregoing display types, and retain their components in reduced and probably functionally ineffective forms. In addition there is : f. C r y p t i c B e h a v i o u r, associated with highly developed mimetic procryptic colouration. The insects possessing it are strikingly inert even when strongly stimulated. 2. The display types do not correspond to the taxonomic divisions within either of the two groups, but they are strongly correlated with colouration type, thus: G e n e r a l i s e d E s c a p e B e h a v i o u r - generalised cryptic colouration R h y t h m i c D i s p l a y s- Generalised cryptic colouration and conspicuous patterns S u s t a i n e d S t a t i c D i s p l a y s, Type 2 - Mimetic patterns (wasp mimicry) Type 1 - Eyespot patterns M i x e d D i s p l a y s - Eyespot patterns V e s t i g i a l D i s p l a y s - These either relate to the total reduction and partial elimination of all pattern elements, e.g. Eudyaria venata, Lobobunaea epithyrena, or to the replacement of one pattern type by another, e.g. Epiphora atbarensis sudanica and Aglia tau. C r y p t i c B e h a v i o u r - Specialised procryptic colouration. 3. There is evidence to suggest that the displays have been evolved from the preparatory movements of flight given by moths which are too cold to fly. Supporting this hypothesis is the fact that those moths whose wing movements during display most closely resemble those of flight, also incorporate the greatest number of other components directly reminiscent of flight in their displays. 4. There is also evidence to support the hypothesis that, in the course of the phylogenetic history of the groups, the displays have replaced each other in the following order: Inadequate Flight Movements - Rhythmic Displays - Mixed Displays - Type 1 Sustained Static Displays - Type 2 Sustained Static Displays - each stage giving rise independently to Vestigial Displays. The Automerinae provide the clearest evidence for such a succession. The position of Generalised Escape Behaviour in this sequence is obscure. It is not considered to be the primitive mode of protective behaviour in the two groups. Nor is the evolutionary status of Cryptic Behaviour known. 5. The experimental evidence concerning the efficacy of the different types of protective behaviour is reviewed briefly. It allows the display types to be arranged into a rough order of increasing efficiency. This postulated order is identical with the order in which the display types appear to have replaced each other in the course of evolution.