|Danaidae - General characteristics|
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The Danaids are represented by 6 genera (groups) in Sri Lanka.
The male also has another organ that is used during courtship - a yellow brush-like organ that is usually tucked inside the last segment of the abdomen. When extruded during courtship, it forms a beautiful globular structure with very fine hairs. These hairs, called 'Hair Pencils', are sometimes rubbed or brushed against the antennae of the female to invoke a cooperative response. The chemicals emitted from this organ contain, among other things, a toxin called danaidone. Females prefer to mate with males who exude significant amounts of danaidone from their hair pencils. In addition, the spermatophore (sperm package) that the male passes over to the female during mating also contains danaidone as well as sperm and other nutrients. The danaidone is incorporated into the eggs as well as into the female's tissues. This not only increases the chances of survival of the eggs that are laid but also makes the female less palatable to predators.
Interestingly, danaidone is not available from the plants that the larvae feed on and must be obtained by the adults after emergence. And this task is accomplished by feeding on plants that do contain these substances. Two widely distributed plants that contain danaidone precursors (often lumped together and called pyrollidizine alkaloids) are Heliotropium indicum and Crotalaria palida. These are annuals that come up with the onset of the rains, and are quite common in the dry and intermediate zones of the island. It is principally from these two plant species that the male Danaids obtain most of the chemicals required to synthesize their pheromones. All Danaids are irresistibly attracted to these plants. The dead or damaged plant parts of Heliotropium indicum are preferred to undamaged plants. In the case of Crotalaria pallida, the developing pods are preferred over all other plant parts. The chemical that is so gleefully sought after by these butterflies has now been identified as lycopsamine. Studies elsewhere have shown that males that do not accumulate danaidone are consistently refused by the females. So it seems that the females not only seek chemicals for their own survival and their progeny, but also indirectly select for genes that are correlated well with good foraging ability; an excellent strategy for survival of the species.
The mimic not only looks like the model, but also behaves like it. Because this disguise increases the chances of the mimic being mistaken for the unpalatable model, it is avoided by its predators. Bates was the first person to describe this type of mimicry, which is referred to as Batesian Mimicry. For this strategy to succeed, the models must be found in small numbers otherwise they will be caught more frequently and the predator will soon uncover their disguise. The female Common Palmfly and female Danaid Eggfly are both Batesian mimics of the Common Indian Crow.
Another strategy that has evolved among butterflies is for all members of a group to resemble each other and also be unpalatable. This approach spreads the chances of any one butterfly been eaten over a lager number of species, and over a lager number individuals within a species. When a bird catches any one of these individuals, it quickly learns to keep away from all the species within the group. This type of mimicry is referred to as Mullerian mimicry, after Muller who first described it. In Sri Lanka, there are only 2 Mullerian groups and they are both within the Danaidae. One group consists of the Common Indian Crow, Brown King Crow and the Double branded Crow. The other group consists of the Blue Tiger, Dark Blue Tiger, Blue Glassy Tiger and Glassy Tiger.