Hawk moths

master fliers and specialized pollinators

Bedstraw hawk moth (Hyles gallii) Photo by Bob Armstrong

If you walk through a field of fireweed, you might see spittle bugs and aphids and –if you are lucky—a hawk moth. They hover at flowers on their rather narrow wings, extending their ‘tongues’ to extract nectar. They can fly very fast, which may have given them the common name of ‘hawk.’ They’re also called sphinx moths: when a caterpillar is at rest, it raises the front part of its body and tucks the head down; someone with a vivid imagination saw a resemblance to the famous Egyptian Sphinx.

There are well over fourteen hundred species of hawk moth in the world. Some of them specialize on extracting nectar from orchids and often incidentally (to the moth) pollinating them. Many hawk moths have long proboscides (‘tongues’), suitable for extracting nectar from the long nectar spurs of certain flowers. When there is a good fit between the length of the spur and the length of the proboscis, the moth can pick up pollen on its eyes or face and transfer it to another orchid flower. If the proboscis is too short, the moth can’t reach the nectar in a long spur and is not likely to visit that kind of orchid very often. If the proboscis is too long, the moth’ head or body will not contact the place in the flower where the pollen is produced, so although the moth can steal some nectar, pollination is unlikely (unless it happens that the pollen is contacted by the proboscis itself).

Certainly the most famous of these relationships (as I mentioned in an earlier essay) concerns the Madagascar star orchid, whose prodigiously long (eleven inches or so) nectar spur caused Darwin to predict the existence of a suitable moth with an equally long proboscis. Sure enough, someone else eventually found that predicted moth, in action. Now there are rumors that a second species of long-tongued hawk moth can also reach the nectar and do some pollination of this orchid.

A little closer to home, in the swamps of Florida and Cuba, the ghost orchid lives high up on tree branches. The nectar spur is said to be about five inches long (but variable) and it is now known that several species of hawk moth can pollinate this species. Unfortunately, the orchid is now quite endangered, in part because of over-collecting by too-avid horticulturalists.

Slightly closer to Alaska, in the tall-grass prairies, the fringed prairie orchids are pollinated by hawk moths. The western fringed prairie orchid has a nectar spur over two inches long, which is said to be longer than most other North American orchids. It is known to be pollinated by four species of native hawk moths (perhaps more) and by one non-native species (that was introduced to North America from Eurasia to help control an invasive weed). This species of orchid is designated as ‘threatened’, largely because of habitat loss as the prairies were plowed under for agriculture. But in addition, the moths are at risk from pesticides drifting over from the agricultural fields. Some published accounts say that no seed set is accomplished in the absence of moth pollination, but others say that a little self-pollination without the help of the moths is possible. In either case, reproduction is generally poor.

In Alaska, little seems to be known about the relationships of hawk moths to flower pollination. Of the seven species on record in the state museum, only some are represented by more than a few specimens (thanks to the helpful entomologist, Derek Sikes, for this info!). I’ll summarize a bit about three of them. Here in Juneau, we sometimes see the bedstraw hawk moth (Hyles gallii) as the adults visit fireweed and other flowers. Two local orchid-watchers have seen and photographed this moth visiting the white bog orchid, carrying pollen on its fairly long proboscis (about an inch long). Is this moth a regular pollinator of this orchid? Hawk moths elsewhere are known to pollinate related species of orchid, but it has been thought that this orchid is pollinated mostly by moths that sit on the flower while they sip nectar (instead of hovering, as a hawk moth does). In any case, hawk moths seem to be uncommon around here, so their role in pollination may be occasional at best.

Another hawk moth that we occasionally see here is called the hummingbird hawk moth (Hemaris thysbe). The wings are clear, without any colorful scales. This species is known to visit many kinds of flowers, including some orchids similar to the white bog orchid, but how many of these visits accomplish pollination is not known. The proboscis is of medium length (less than an inch), so deep nectar sources are not available to this moth.

One other fairly well-represented species in the museum collection is the one-eyed sphinx moth (Smerinthus cerisyi), which sports pretty blue eyespots on the hind wings. Although I don’t know if it has been recorded in Juneau, there are records from coastal British Columbia and from near the head of Lynn Canal, so it seems possible that we might see it here. It has an extremely short proboscis (only a few millimeters long), and one source states that it is not functional at all. In that case, the adults would not feed and there would no pollination.

Hawkmoth caterpillars are often called horn worms, for the horn-like projection that sticks up from the rear end. The three Alaska species that I’ve mentioned all have ‘horns’, although not all hawk moth caterpillars do. The caterpillars are herbivorous, commonly eating a variety of leaves. Bedstraw caterpillars eat fireweed, plantain, enchanters nightshade, and many other things, in addition to bedstraw. Hummingbird caterpillars eat snowberry, blueberry, cherry, thistle, clover, and more. One-eyed sphinx caterpillars forage chiefly on willow and poplar, but occasionally other species too.

Visiting new territory

dabbles in California natural history

I recently made a visit to California, stopping briefly in Tahoe and the region around San Jose. There was little opportunity to pursue my usual natural history interests, but here is a small collection of observations.

–A chubby California ground squirrel climbed up a manzanita shrub to eat the fruits that give the shrub its name (manzanita means ‘little apple’).

–A family of Brewer’s blackbirds scoured the ground for picnic scraps, the well-grown juveniles still trying to coax tidbits from the parents. These blackbirds were the common avian scroungers in many picnic areas, waiting for visitors to drop tasty bits. Other moochers around the picnic tables included one or two kinds of chipmunks and the golden-mantled ground squirrel, which looks like a big chippie but lacks the head stripes.

–A group of wild turkeys was a big surprise, seen as we whizzed along a highway. It turns out that turkeys were introduced to this area decades ago from the native Rio Grande population.

–A male western tanager posed next to the road, so everyone in our slow-moving cavalcade got a good look at it. Even the non-birdwatchers enjoyed this colorful fellow.

–Both ponderosa pines and Jeffry pines grow at mid elevations here. They look quite similar, but their the spines on their hefty cones are different: those on ponderosa cones stick out and stab the hand that grabs them, but those of Jeffrey cones are bent back and do not stab. If you can’t find a cone for conducting this test, try smelling the bark. Jeffrey pine bark is aromatic, fruity or vanilla-scented, depending on the observer. My nephew and two of his small offspring joined me in smelling the tree trunks; the rest of the clan took this as confirmation of lunacy.

–On a stroll through a coastal redwood grove, I noted that many of the big trees were circled by dense crowds of small juveniles sprouting at the base of the trunk. Redwoods sometimes form hefty burls at the root crown. This type of burl is known specifically as a lignotuber, composed of numerous dormant buds and food reserves. The buds are reportedly stimulated to sprout if the adult is damaged or begins to die. If the adult actually dies, a few of the sprouts can take over that location, surrounding the old stump. However, unlike the very thick-barked adult, the sprouts would be very vulnerable to ground-fire. Burl sprouts also self-thin rapidly, especially at low light levels; at extremely low light levels, all of them die. Presumably, the crowds of juveniles around these trees had not self-thinned much: some of the small trees were several feet tall, despite the crowding.

–In the same grove, a few of the small redwoods had white, not green, foliage. Lacking chlorophyll, these cannot conduct photosynthesis to produce the carbohydrates needed for growth. So they are dependent on their connections to adjacent trees that do have the usual green foliage. Those connections might be direct (root to root) or via fungal links. The white trees are little parasites!

–The Monterey Bay Aquarium occupied me for several hours, gawking happily at the varied displays. A large, tropical comb jelly was fancifully adorned with more colorful frippery and furbelows than any I’d ever seen before (being accustomed to more modest ones, with their rows of cilia around an oval body). The perilous state of the endemic population of vaquitas in the Gulf of California was featured in one area. These small porpoises get caught in fishing nets and drown; they also have already lost most of their food fishes to greedy humans, who ship just a certain body part to China (for pseudomedical sales). Sadly, there are only thirty or fewer still left.

A video of cuttlefish showed the tentacles shooting out with incredible speed to snag a hapless little shrimp. That fascinating sight captivated me, but I don’t know much about cuttlefish, except that they are related to octopuses, nautiluses, and squid, a group of molluscs collectively known as cephalopods. So I looked up a few bits of information. The two long tentacles are hidden among the eight short arms, when the animal is just cruising around. Most cuttlefish are fairly small, with a body length less than about twenty inches long. They travel by jet propulsion (forcing water out through a siphon) and by undulating the fins on the side of the body; the ones I watched also seemed to walk on some of the arms (but that might have been an illusion). Cuttlefish can change color and pattern rapidly and spew forth a cloud of ink to obscure their movements. They have relatively large brains and excellent vision. They don’t live very long, reportedly just a couple of years. Males fight over females and the winner inserts his sperm into the front of the female’s mantle cavity using his tentacles. Males can even remove the sperm of the female’s previous mate, flushing it away (a tactic not limited to these critters, by the way).

And here are some un-natural history observations: It was a short visit, but it didn’t take long to grow weary of congested highways and traffic delays, the many other roads squirming off in all directions (GPS was essential), beaches that were packed with bodies in various stages of sunburn, parks thronged with humans. Almost every flat expanse that wasn’t covered with buildings featured vast agricultural fields of sunflowers or artichokes or other monocultures. Gigantic strawberry fields harbored odd, round-backed creatures near every row—stoop-labor at work. Some enormous fields were entirely sheathed in plastic, reportedly for weed control. Not my idea of a place to live! I was very glad to leave it and come Home!

Hiding in plain sight

camouflage is more than just visual

When the presence of an animal is difficult to detect or it is hard to locate, it is said to be cryptic. Think of a female mallard sitting on her nest; her mottled brown feathers are good camouflage—they make her look like a little heap of old leaves, not readily distinguishable from the background. We may actually be seeing her but be unable to register her presence; we just don’t detect her, much less locate her nest. She is not out of sight, like bunny in a burrow; she is hidden plain sight.

A famous case of background matching was very well documented. The peppered moth (Biston betularia) in the UK likes to rest on lichen-covered tree trunks and branches. The typical form of this moth has white wings speckled with black spots. The wings also reflect UV light, but even that has a speckled pattern. The speckled pattern makes the resting moth almost invisible as it sits on the tree bark.

Photo by Bob Armstrong

During the Industrial Revolution in England, beginning in the mid 1800s or so, pollution from factories killed off many of the lichens and the tree bark became blackened with soot. Then the typical form of the moth was very conspicuous against the dark bark and they were readily picked off by hungry birds. A mutant form with black wings, much less noticeable on the sooty trees, then became more common. Natural selection in action!

More recently, industry has become environmentally somewhat cleaner, the trees are not so dark, and the black form is vulnerable to predation. As a result, the white, typical form of the moth is again becoming common.

Because we humans are so heavily oriented to visual things, we usually think of visual cryptic-ness (or crypsis). But why not think of crypsis that relates to other sensory systems? There are several studies suggesting that there may acoustic or olfactory crypsis, and hints for other sensory modalities.

–Acoustic signals are used by many animals for courtship or other communication among individuals of the same species. For instance, males of a neotropical frog make complex calls to attract females, especially when several males are calling simultaneously. But in the presence of predatory bats, they simplify their calls, even though the females prefer the complex ones. At the price of reduced attractiveness, the simpler call appeared to make the male callers less detectable to the bats. They didn’t completely stop calling—which would be acoustically hiding. They just became less detectable.

Many small birds produce a high-frequency, thin alarm call (“seet”), which does not carry very far. This is thought to be more difficult to locate than the usual contact or distress calls. Experiments with captive hawks indicated that these predators had trouble locating the source of the sound and attacked less often, when the potential prey used the high-frequency calls. The small birds can hear the call and be on the watch for trouble, but they don’t need to know the exact location of the caller.

–The leaf-eating caterpillars of another moth (that’s closely related to the peppered moth) look a lot like little twigs, which might protect them from birds but not from predatory ants that hunt primarily by smell. Yet such ants just walked over the caterpillars as if they weren’t there at all and didn’t attack them. However, if the caterpillars were transferred to a different food plant, the ants attacked them readily. Then, after the caterpillars had fed on the new food plant for a while and molted to the next larval stage, they were again ignored by the ants. In this case, the protection comes from matching the chemical constitution of the food plant.

Some elaborate studies of herbivorous pine bark beetles compared the chemical signals and the responses of predatory beetles in different areas (California and Wisconsin). Pine bark beetles use chemical signals called pheromones to attract others of the same species. Predatory beetles cue in on those signals. As might be expected, California pine bark beetles liked their own pheromones better than the Wisconsin ones, and vice versa. But the California predators homed in better on the Wisconsin pheromones, and the Wisconsin predators homed in on the California pheromones better than on the local ones of each population. In other words, the prey were less apparent to local enemies than distant ones, suggesting they were chemically somewhat cryptic.

Birds have a preen glad just above the tail that produces oils and waxes that the birds use to dress their feathers. A fascinating study revealed that the normal waxes are replaced by less volatile waxes during the nesting season of several ground-nesting shorebirds. Furthermore, if only the female of these species does the incubating, then only the females show the shift in wax composition. Ground-nesting birds are particularly vulnerable to predation by mammals, and experiments with a search dog trained to the smell of these waxes showed that the dog was less able to detect the waxes produced in the nesting season.

–In addition, there are hints—but only hints—that shifts in electrical fields, or in production of wakes by swimming animals, or in substrate vibrations have the potential to be used by prey animals to whom crypsis could be advantageous. There are, potentially, many ways to be hidden in plain ‘sight’. And many investigations are needed. In the meantime, it is useful and interesting to contemplate things beyond our usual ken.

A big thank you to the gracious CBJ librarians who promptly obtained a necessary reference for me!