ONTOGE NY OF DEFENSE AND ADAPTIVE COLORATION IN LARVAE OF THE COMMA BUTTERFLY, POLYGONIA C-ALBUM (NYMPHALIDAE)

In many butterfly species early and late larval instars differ in coloration. The first three lmval instars in the COmma butter11y, l'olygonia c-album, have coloration thal, lo the human eye, appears to be disruptively cryptic. The last two instars, which are defended by strong branching spines, are instead strikingly colored in white, black, and orange. It has been suggested that this coloration is also cryptic, mimicking bird droppings. We test the idea that lhis coloration is instead (or Simultaneously) aposematic, using young chickens as models of bird predators. Some chicks readily ate third-instar larvae, both initially and after having tasted them, suggesting that no chemical defense is present in comma larvae. The chicks also ate dcad fifth -ins tar larvae from which the spines had been removed. Chi<:ks initially attacked fifth-instal' larvae with intact spines, but learned to avoid them already after the first attack This suggests an aposematic flmction of the coloration, which can have evolved by individual selection in this and many other nymphalid species with spiny larvae, because larvae were not harmed during the learning process. The evolutionary causes of the ontogenetic shift in defense tactics are discussed. Additional key words: aposematisrn, mirniery, life histolY, predation, adaptive function. Adaptive coloration in animals has been a very active research field in evolutionary biology over the years (e.g., Poulton 1890, Cott 1940, Kettlewell 1973, SillenTullberg 1988, Malcolm 1990), and one in which the Lepidoptera have always featured prominently as model species. Adaptive coloration includes crypsis (helping to avoid detection by other animals), signalling, and thermoregulation. Crypsis can be achieved by having colors similar to the background, or by having mosaic patterns of spots or stripes that break up the contour and surface area of the animal (called "disruptive crypsis"; Cott 1940, Edmunds 1990). Signals can be of several types, including "aposematism," defined as conspicuous colors advertising unprofitability to predators (Guilford 1990a, b). A phenomenon that may include aspects of both Signalling and crypsis is "mimicry," a term used for organisms that adaptively resemble another species, as in the cases of Miillerian and Batesian mimicry (Malcolm 1990). In the former case, unpalatable aposematic organisms mimic each other. In the latter case, a palatable organism mimics an aposematic one, capitalizing on its defenses. Sometimes the term mimicry is extended to cases when organisms mimic the shape and colors of objects or inlmobile organisms for the purpose of crypsis , for example as in stick insects (Edmunds 1990). Finally, variation in eoloration can be due to different requirements for thermoregulation rather than predator avoidance (e.g., Shapiro 1976, Kingsolver & Watt 1983). For all of these types of (presumably) adaptive coloration, it is probably fair to say that the function is often assumed, but seldom tested. J Email: Soren.Nylin@zoologi.suse Tel: +46-8-164044 Fax: +46-8-167715. The eoloration of the first three larval ins tars in the comma butterfly, Polygonia c-albl1m L. (Nymphalidae), appears to the human eye to be disruptively cryptic (Fig. 1A), at least when seen against a naturally variegated background of light and shadow. The last two instars, which are defended by strong branching spines, are much more strikingly colored in white, black and orange (Fig. 1 B). The same type of coloration is present in the probable sister species to P c-albl1m, the N earctic P jal1nl1s (Scudder 1889, Scott 1986). It has been suggested that this type of coloration is also cryptic, the large continuous white area supposedly mimicking bird droppings (Thomas 1986). Resemblance to bird droppings is seen in many butterfly larvae, e.g., in Limenitis and Papilio (Scott 1986), although to our knowledge the function has never been tested. Late instar larvae of other Polygonia species (e.g., P interrogationis, P comma, P satyrus, and P c-aureum) and the related Kaniska canace are Similarly colored in conspicuous white, black and orange, but with no continuous white areas (Scott 1986, Teshirogi 1990). This suggests the possibility that the coloration is aposematic in these species, and then perhaps in Pc-album and P jal1nus as well. Here, we study the function of the coloration of late ins tar larvae of Pc-album using young ehickens as models of bird predators. For comparison and as a control for effects of novel food, we also tested the chicks with third-ins tar larvae. MATERIALS AND METHODS Females of P c-albl1rn were captured near Akersberga, north of Stockholm, Sweden. Eggs were obtained in flight cages where the host plant Urtica dioica (U rticaceae) was present. Larvae were reared on this plant in the laboratory. Several asynchronous rearings 70 JOURNAL OF THE LEPIDOPTERISTS' SOCIETY