Arched Hook-tip Moth
This beautiful Arched Hook-tip (Drepana arcuata) landed on the house recently, a new species for the porch-light list, and now the BugLady wants to meet its caterpillar. She may still have a chance at it—the AHT has two generations (two broods) each summer here in God’s Country, and the second-brood caterpillars should afoot any time now. Pupae of the first generation become adults fairly quickly, but pupae of the second generation overwinter and emerge early next summer.
AHTs belong in the “Hooktip and False Owlet moth” family Drepanidae (from the Greek word drepana meaning sickle), also a newcomer to BOTW. There are about 660 species in the family worldwide (only 8 in the eastern U.S.A.). Drepanid moths are medium-sized moths (wingspread 1” – 1 ½”) that have uniquely-formed hearing organs, and many (but not all) have hooked wing tips. In many species, the proboscis that is short or absent, but the AHT moth does feed. North American members of the family can be found throughout the continent except for the far (far) north—Alaska and the Yukon Territory.
According to the range map on the mothphotographers website, the AHT is largely missing from the Great Plains and the Gulf Coast but is present across Canada. Based on the pictures on that site and on bugguide.net, most adult AHTs are paler than this sable-colored specimen (Borrer calls them “dirty white”). The BugLady read in one source that the members of the summer brood are darker in color.
Arched Hook-tip Caterpillars
AHT caterpillars have their own name—the Masked birch caterpillar (their host plants are paper birch and several species of alder). Like other members of their family, AHT caterpillars have reduced/fused rear legs, so the abdomen tapers dramatically to the rear. Wagner, in Caterpillars of Eastern North America, says that they often hold up both head and tail, looking like dragons. Female AHTs lay a half-dozen eggs per leaf, and the caterpillars live and feed communally for the first 10 days or so. They spin silken shelters on a leaf surface, often drawing several leaves around them, and they leave their webby homes at night to nibble at the leaf’s edge. Caterpillars will consume several leaves before they pupate, moving from “used” leaves to new ones (they keep the nest cleared of frass (bug poop) and they sever the petiole (leaf stem) of a fed-upon leaf so as not to signal birds). They never move far from where the eggs were laid, and they pupate in a cocoon hidden within a folded leaf.
So, what are the AHT’s claims to fame, and why does the BugLady want to find a caterpillar?
First, the moth’s “ears.” Lots of insects have hearing organs (tympana), which may be located on various parts of an insects’ anatomy. Depending on the family, ears might be found on the thorax or abdomen, or even the wings or mouthparts (in Drepanidae, the ears are located toward the front of the abdomen). The ears of nocturnal moths detect the frequencies of bats, but in some species, they are also receivers for communication between individuals. Ears are popular in the Lepidoptera (even some butterflies, which are diurnal and have no need for bat-detectors, have them, though most butterflies do not produce sound. They are thought to be tuned to the frequency of bird wing-beats). The ears of Drepanid moths are structurally more complicated than those of most other insects, though they’re used to detect the same sounds.
Second, the caterpillar. Even though they live a sedentary lifestyle within those webbed leaves, AHT caterpillars have developed a number of ways to communicate with perceived threats in their environment. The biggest threat comes from AHT caterpillars that are “in between” nests, and figure that they might evict a nest-owner (building web nests is expensive, energy-wise). Researchers Yack, Smith, and Weatherhead studied AHT caterpillars (a project that was initiated when Jayne Yack, who was rearing some caterpillars in her dining room, heard their vibrations in the quiet room). Here’s what they found:
- AHT caterpillars generate vibrations (acoustical displays) into their substrate (leaves) using both their mouthparts and specialized structures at the tip of the abdomen (anal oars). Acoustical displays take the place of more dangerous physical altercations.
- AHT caterpillars are territorial and defend their nests against other AHTs. When it sees another AHT caterpillar on its leaf, the resident stops feeding, retreats into its shelter, and starts signaling when the intruder is about 2” away.
- This behavior is somewhat ritualized and consists of first, a rasping sound produced by drawing the anal oar forward on the leaf surface; second, “staccato drumming of the serrated ‘incisors’ of the open mandibles;” and third, a “loud,” rasping sound made when the mandibles are drawn across the leaf’s surface.
- It’s thought that they use these vibrations to deter both interloper AHT caterpillars and insect predators, and it’s possible that the caterpillars communicate with each other while still living communally.
- A caterpillar’s world is full of stimuli, and responding to all of them would be exhausting. AHT caterpillars don’t react to substrate vibrations caused by wind or rain, and they can differentiate the sound of another AHT caterpillar approaching on a windy day.
- AHT “dust-ups” don’t last very long (just minutes—it’s counterproductive to send all that sound out into a world full of predators), and the homeowner typically (86% of the time) wins. The bigger the intruder in relation to the resident AHT, the longer the contest will be and the more likely that the intruder will be successful.
- Very infrequently, the contest deteriorates into head-butting; far more frequently, the interloper damages the nest by biting through it.
For more info, check:
NBC News.com’s Civilized caterpillars talk with their butts
The BugLady has always wondered about how an organism that engages in this kind of non-combat “face-off” figures out whether it’s bigger or smaller than its opponent (after all, it’s never seen itself in a mirror). Some sort of sense of “self-image?” Do bugs get “psyched out?”