| Hawk moths | |
|---|---|
| Hummingbird hawk moth Macroglossum stellatarum |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Order: | Lepidoptera |
| (unranked): | Macrolepidoptera |
| Superfamily: | Bombycoidea |
| Family: | Sphingidae Latreille, 1802 |
| Type species | |
| Sphinx ligustri Linnaeus, 1758 |
|
| Subfamilies | |
| Diversity | |
| About 200 genera, roughly 1,200 species |
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Sphingidae is a family of moths (Lepidoptera), commonly known as hawk moths, sphinx moths and hornworms, that includes about 1,200 species[1]. It is best represented in the tropics but there are species in every region[2]. They are moderate to large in size and are distinguished among moths for their rapid, sustained flying ability[2]. The narrow wings and streamlined abdomen are adaptations for rapid flight.
Some hawk moths, like the hummingbird hawk moth, hover in midair while they feed on nectar from flowers and are sometimes mistaken for hummingbirds. This hovering capability has evolved only three times in nectar feeders: in hummingbirds, certain bats, and these sphingids[3] (an example of convergent evolution). Sphingids have been much studied for their flying ability, especially their ability to move rapidly from side to side while hovering, called 'swing-hovering.' It is thought that this evolved to deal with ambush predators that lie in wait in flowers[3].
Sphingids are some of the fastest flying insects, some are capable of flying at over 50 km/h (30 miles per hour). They have a wingspan of 35-150 mm.
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Most species are multivoltine, capable of producing several generations a year if weather conditions permit[4].
Females lay translucent greenish, flattened, smooth eggs[2]. Eggs are usually laid singly[1] on the host plants[4]. Egg development time varies highly, from 3 to 21 days[4].
Sphingid caterpillars are medium to large in size, with stout bodies. They have 5 pairs of prolegs[4]. Usually their bodies lack any hairs or tubercules, but most species have a "horn" at the posterior end[2], which may be reduced to a button, or absent, in the final instar[4]. Many are cryptic greens and browns, and have countershading patterns to conceal them. Others are more conspicuously coloured, typically with white spots on a black or yellow background along the length of the body. A pattern of diagonal slashes along the side is a common feature. When resting, the larva usually holds the legs off the surface and tucks its head underneath, which gives rise to the name 'sphinx moth'[4]. Some tropical larvae are thought to mimic snakes[2]. Larvae are quick to regurgitate their sticky, often toxic, foregut contents on attackers such as ants and parasitoids[4]. Development rate depends on temperature, and to speed development some northern and high altitude species sunbathe[4]. Larvae burrow into soil to go into chrysalis, where they remain for 2–3 weeks before they emerge as adults.
In some sphingidae, the pupa has a free proboscis, rather than being fused to the pupal case as is most common in Macrolepidoptera [2]. They have a cremaster at the tip of the abdomen[4]. Usually they pupate off the host plant, in an underground chamber, among rocks, or in a loose cocoon[4]. In most species, the pupa is the overwintering stage.
Antennae are generally not very feathery, even in the males[2]. They lack tympanal organs but members of the tribe Choerocampini have hearing organs on their heads[2]. They have a frenulum and retinaculum to join hindwings and forewings [2]. The thorax, abdomen, and wings are densely covered in scales. Sphingids may have a reduced proboscis, but most have a very long proboscis[2]. They use it to feed on nectar from flowers. Most are crepuscular or nocturnal, but some species fly during the day[4]. Both males and females are relatively long-lived (living 10 to 30 days)[4]. Prior to flight, most species shiver their flight muscles to warm them up, and, during flight, body temperatures may surpass 40°C ([4].
In some species, sexual dimorphism (differences in form between the sexes) is quite marked. For example, in the African species Herse convolvuli (the Convolvulus or Morning Glory Hawk Moth), the antennae are thicker and wing markings more mottled in the male than in the female. Only males have both an undivided frenular hook and a retinaculum. Also all male hawk moths have a partial comb of hairs along their antennae.[5] Females call males to them with pheromones. The male may douse the female with a pheromone[4] before mating.
Some species fly only for short periods either around dusk or dawn, while other species only appear later in the evening and others around midnight, but such species may occasionally be seen feeding at flowers during the day. There are a few common species in Africa, such as Cephonodes hylas virescens (the Oriental Bee Hawk), Leucostrophus hirundo and Macroglossum trochilus, which are diurnal[5].
Sphingid larvae tend to be specific feeders, rather than generalists[4]. Compared to similarly sized saturniids, sphingids eat soft young leaves of host plants with small toxic molecules, and chew and mash the food into very small bits[6]. Some species can tolerate quite high concentrations of specific toxins. Tobacco hornworms, Manduca sexta, detoxify and rapidly excrete nicotine, as do several other related sphinx moths in the subfamilies Sphinginae and Macroglossinae, but members of Smerinthinae that were tested are susceptible[7]. The species that are able to tolerate the toxin do not sequester it in their tissues; 98% was excreted. However, other species, such as Hyles euphorbiae and Daphnis nerii do sequester toxins from their hosts, but do not pass them on to the adult stage[4].
Most adults feed on nectar, although a few tropical species feed on eye secretions and the Death's-head Hawkmoth steals honey from bees[4]. Night-flying sphingids tend to prefer pale flowers with long corolla tube and a sweet odour, a pollination syndrome known as 'sphingophily'[3]. Some species are quite general in visitations, while others are very specific, with the plant only being successfully pollinated by a particular species of moth[3]. Orchids frequently have such specific relations with hawkmoths, and very long corolla tubes. The Comet Orchid, Angraecum sesquipedale, a rare Malagasy flower with its nectar stored at the bottom of a 30 cm long tube, was described in 1822 by Aubert du Petit-Thouars, and later Charles Darwin famously predicted that there must be some specialised animal to feed from it:
"[A. sesquipetale has] nectaries 11 and a half inches long, with only the lower inch and a half filled with very sweet nectar [...] it is, however, surprising, that any insect should be able to reach the nectar: our English sphinxes have probosces as long as their bodies; but in Madagascar there must be moths with probosces capable of extension to a length of between 10 and 12 inches!"[8]
Alfred Russel Wallace published a sort of "wanted poster" (properly, a drawing in a book) [9] of what this butterfly might look like, and, concurring with his colleague, added:
"[The proboscis of a hawkmoth] from tropical Africa ([Xanthopan] morganii) is seven inches and a half. A species having a proboscis two or three inches longer could reach the nectar in the largest flowers of Angræcum sesquipedale, whose nectaries vary in length from ten to fourteen inches. That such a moth exists in Madagascar may be safely predicted, and naturalists who visit that island should search for it with as much confidence as astronomers searched for the planet Neptune, – and they will be equally successful."[10]
A possible species was discovered 21 years later, for which the hawkmoth in question was found and described as a subspecies of the one mentioned by Wallace: Xanthopan morganii praedicta[11], for which, the subspecific name praedicta ("the predicted one") was given. The subspecies has however been subsequently declared as invalid due to its similarity to mainland species.
There are around 1200 species of hawk moth, classified into around 200 genera. Some of the best known species are:
John Linnell, of the rock band They Might Be Giants, reportedly wrote the song "Bee of the Bird of the Moth" (on their album The Else) after he saw a "hummingbird moth", presumably one of the members of this family that resembles a hummingbird.[12]
Edgar Allan Poe includes a sphinx moth in his short story, "The Sphinx". The main character mistakenly thinks that the moth on a window is a huge monster. Much to his surprise, his friend points out that it is in fact very close and not on a hill in the distance.
