Sphingidae

Introduction

Hawkmoths have in the past been placed in a superfamily of their own but are now generally considered to be members of the Bombycoidea. The group includes about 1350 species, and although best developed in the tropics it is represented worldwide. The most conspicuous aspect of sphingid biology is the capacity of many of the moths for fast, long-distance, and often migratory flight. The moths are appropriately shaped with narrow wings and an aerodynamic abdomen. Many species have evolved the ability to hover in front of flowers to feed in a manner resembling that of hummingbirds. Hawkmoth larvae are large, often marked by stripes or eyespots, and normally have a conspicuous horn at the rear (in the United States sphingids are called hornworms).

Adult

The antennae are usually fasciculate (ventrally compressed with elongated hairs) in the males and filiform (threadlike with very short hairs) in the females but occasionally are bipectinate (with lateral extensions making the antenna resemble a feather). Sometimes the antenna is swollen distally and hooked at the apex. The proboscis is usually well developed, and sometimes extremely long, but may be reduced and non-functional. The maxillary palps are very small and 1-segmented, and the labial palps are prominent, ascending, and appressed to the head. On segment 1 of the labial palp, the inner surface may bear fine, dense hairs. Segment 2 is sometimes swollen and acts, together with a structure called the pilifer, as a non-tympanate hearing organ. An epiphysis, a spur used for cleaning the antennae, is present on the foreleg. There is also a pair of spurs on the midlegs and two pairs on the hindlegs (although one of these pairs may occasionally be absent). The midleg often bears a row of slender setae, known as the midtarsal comb, on the first tarsal segment. The forewings are typically considerably longer than broad, and the hindwings only about twice as long as broad. The pattern of veins in the wings is important in classification (see Scoble 1995 for an explanation of terminology) In the forewing, veins R1 and Rs2 are stalked or fused, and M1 arises from the same point as Rs or is stalked with it. In the hindwing, R1 diverges from a point about halfway along the cell and meets Sc. CuP is absent from both the fore- and hindwings. The fore- and hindwings are coupled by a single (males) or several (females) bristles arising from the leading edge of the hindwing (frenulum) that lock into a curved bar (retinaculum) on the forewing, but the system is occasionally reduced. The forewing-metathoracic locking-device ("haftfeld") found in other moths is absent. The abdomen is robust, but narrows posteriorly. There are no metathoracic or abdominal tympanal hearing organs.

Immature stages

The eggs are generally translucent green, and rounded or slightly flattened. The surface is not strongly sculptured.

The larvae are medium-sized to large. Hairs may be present, but they are not usually conspicuous. A prominent caudal horn usually occurs on abdominal segment 8, although the structure is sometimes rudimentary. Rarely, a complete set of scoli or spines is present on the body. A full complement of prolegs is present, and the crochets (hooks) are arranged in a row of two alternating lengths. Prominent eyespots may occur on the thorax.

The pupa is fusiform (torpedo-shaped) and has prominent hooks (the cremaster) at the posterior end. To accommodate the long adult tongue, the pupal proboscis is often looped or coiled away from the body and the adult tongue turned back on itself to terminate in the usual position between the wings.

Biology

Adults are generally nocturnal or crepuscular, and there are few truly diurnal species. Both males and females are generally long-lived and strong fliers. Sphingidae, in contrast with Saturniidae (the emperor silk moths) feed over a long period, possess vigorous flight mechanisms, and have females that lay eggs singly, rather than in large batches. Many hawkmoths drink nectar from flowers while hovering in front of them, although some, notably the Bee Hawkmoths (Cephonodes and Hemaris), also hold onto the flower. In Smerinthinae and some other groups, the proboscis is reduced and non-functional, and these insects do not feed from flowers. The long proboscis of hawkmoths, and the long floral tubes of the flowers they pollinate, is often considered to be a classic example of coevolution. Charles Darwin famously predicted the presence of a hawkmoth with a particularly long tongue after studying the structure of a Madagascan orchid.

Warning (aposematic) markings are well known in the larvae. For example, many Choerocampina have pairs of eyespots on the thorax and sometimes also on the abdomen. These markings are displayed in a threatening way when the head and thorax are drawn in and the abdomen bulges. The Neotropical species, Eumorpha labruscae, is said to mimic a small viperid snake, the eyespots on the inflated abdomen representing the eyes of the reptile, and a line on the head its forked tongue. Larvae of most members of the family are however cryptically coloured. The adults of the genus Acherontia (the Death's Head Hawkmoths) make high-pitched squeaking sounds that are said to mimic the piping of a queen bee and to give the moths protection as they rob hives of honey. Members of the Acherontiini, Choerocampina and the genus Xanthopan have ultrasonic hearing organs (they are formed from modified pilifers and labial palps and do not include a tympanum).

Classification

The family is divided into three subfamilies, Smerinthinae, Sphinginae and Macroglossinae. As in many Lepidoptera (and insects more generally), many of the most critical characters for classification involve the structure of the male and female genitalia.

Smerinthinae lack the patch of short, sensory hairs on the inner surface of the first segment of the labial palp and the proboscis is typically reduced . The hindwing often bears an eyespot. In the larva, besides oblique segmental stripes, longitudinal stripes sometimes run along the body. The proboscis of the pupa is not looped away from the body, but is entirely fused with it. There are about 380 species of Smerinthinae, occurring mainly in the Ethiopian and Oriental regions, but some are Holarctic.

Sphinginae is also characterized by the absence of the patch of short, sensory hairs on the inner surface of the first segment of the labial palp, and like Smerinthinae most have symmetrical male genitalia, and female genitalia with the genital plate composed of both lamella antevaginalis and lamella postvaginalis. Most larvae have paired lateral, oblique lines that run from the antero-ventral margin to the postero-dorsal margin of a segment. The caudal horn is generally well developed. Sphingine larvae are usually more granulose or minutely spinose than are those of Macroglossinae. In the pupa, the proboscis is often partly looped away from the body. Most of the 280 or so species occur in the New World, but the subfamily is also found in the Old World.

In Macroglossinae, there is a patch of short sensory hairs, on an otherwise naked patch, on the inner surface of segment 1 of the labial palp; the male genitalia may be symmetrical or asymmetrical; and often the female genital plate is reduced and situated posterior to the ostium. The caudal horn of the larva is often reduced to a short squat structure, and it is within Macroglossinae that eyespots on the larvae tend to be encountered on the abdomen. There are three tribes.

The Dilophonotini include nearly 170 species. Most occur in the Neotropics, but the group is also represented in the Holarctic. The genitalia of both the male and the female are often asymmetrical. The larva is often brightly coloured and bear? black, white, and yellow bands. In the genera Cephonodes and Hemaris, areas of the wings lose their scales shortly after the moths make their maiden flights.

The Philampelini, with 24 species, are predominantly Neotropical. The very slender caudal horn occurring in the first instar larva is replaced with a button-like structure in the last instar.

The Macroglossini are an unnatural grouping representing those Macroglossinae that do not fit into the other tribes. There are about 350 species. The larvae often bear eyespots on the thorax. The subtribe Choerocampina, of worldwide distribution, includes about 250 species. They are characterized by the modification of the labial palps and pilifers into ultrasonic hearing organs. The larvae often bear a row of eyespots dorsolaterally.

For more details of sphingid morphology, biology and classification, with further explanation and examples, see Kitching, I.J. & J.-M. Cadiou, 2000, Hawkmoths of the world: an annotated and illustrated revisionary checklist (Lepidoptera: Sphingidae). The Natural History Museum, London & Cornell University Press, Ithaca. A general text on Lepidoptera, which provided the basis for the above synopsis, is that of Scoble, M.J., 1995, The Lepidoptera. Form, function and diversity. The Natural History Museum / Oxford University Press.