Halobates
- Oceanic Insects
Structure and Function
Nils Møller
Andersen
Zoological
Museum, University of Copenhagen
Ó Copyright, N. M. Andersen & Zoological Museum, University of
Copenhagen
Sea skaters, Halobates, are medium-sized, 3.4-6.5 mm long, dull colored insects but owing to light interference in the hair-layers of their body they appear greyish or silverish. Like other members of the water striders (Hemiptera-Heteroptera, family Gerridae), the antennae are 4-segmented, long, the eyes are large globular, with a multitude of facets. The mesothorax is greatly prolonged and the points of insertion of the long slender middle and hind legs are widely separated from those of the much shorter, robust front legs. The middle and hind coxae are inserted laterad on the body and rotated to an almost horizontal inclination. The claws are inserted pre-apically on the last of two tarsal segments. All marine species known so far are wingless. In Halobates and their allies, the abdomen is greatly shortened. The male genital segments are large, protruding from the abdominal end.
The overall structure of Halobates thus deviates from the generalized insect plan in several ways. Most of these modifications are adaptations towards a life in the water-air interface and especially towards locomotion on the water surface and involve specializations in the thoracic skeleton and musculature, leg structure and surface fine structure of body and legs (Cheng, 1973b, 1985; Andersen, 1976, 1977, 1982; Andersen & Polhemus, 1976).
The fine structure of the body surface has
been studied by scanning electron microscopy (Cheng, 1973a; Andersen, 1977). The
hair pile of the body of Halobates is comprised by two kinds of
macro-hairs inserted in sockets. The first kind is 20-30 µm long, about 1µm
wide at base and inclined at angles of 20-40º. These hairs are evenly
distributed throughout the body surface at densities of 8,000-12,000 per square
mm, forming a regular hair-layer 6-10µm thick. The second kind is slightly
longer, more erectly inserted, with densities of 4,000-5,000 per square mm. In
addition, there is a velvety undercoat of hook-like microtrichia which are 1.5µm
high, 0.5µm wide at base, and 0.6-1.5µm wide at the apex. The base is often
provide with slender outgrowths. The density of these microtrichia is very high,
6-7 x 105 per square mm. This undercoat is absent from the antennae
and legs.
The function of the elaborate body
hair-layers is to protect Halobates from being wetted when occasionally
submerged or sprayed by mist or rain. When a water strider is submerged in water
it carries with it a large bubble of gas entangled in the hair-layers of its
body and appendages. This gas store makes the insect strongly buoyant and it
will rise to the surface rapidly when released. Once on the surface, the water
falls away leaving the insect dry. However, the
When resting, the body of the sea skater is elevated above the water, and only the distal segments of the legs are in contact with the surface film. A water strider weighting 10 mg requires a total line of contact of about 0.4 cm with the water just in order to be supported on the surface. Since many water striders, including Halobates, are able to make vertical jumps from the water surface to a height of several cm (Cheng, 1974) the thrust produced by the legs may reach a magnitude of more than ten times the weight of the insect. The specialized long hairs ensures a corresponding increase in the line of contact (Andersen, 1976).
The three pairs of legs are adapted for
different functions. The short and stout front legs support the body or grasps
and hold the prey during feeding or the female during copulation. The long and
slender middle legs propel the body like oars beating in synchrony; the hind
legs are chiefly used for steering and supporting the body when the middle legs
are lifted off the surface and protracted. The insertion of the middle and hind
legs on the side of the posterior parts of meso- and metathorax allow extremely
wide movements of these legs.
The exact movements of the legs during
locomotion have been studied in some freshwater gerrids by cinematographic
analysis (Andersen, 1976). The middle legs push against the anterior steep side
of a surface wave generated by the insect itself. This requires that the legs
are moved to the rear somewhat faster than the speed with which the wave
spreads. The very long middle legs and the powerful legs muscles make it
possible to obtain a very high angular speed, and in this in turn is a
prerequisite for the use of the water surface in the manner of a starting block.
By this jump-and-slide movement, the water strider may achieve a body velocity
of 80-130 cm per second in a very short time. The following slide phase covers a
distance which is 5-10 times longer.
Andersen NM. 1976. A comparative study of
locomotion on the water surface in semiaquatic bugs (Insecta, Hemiptera,
Gerromorpha). Vidensk. Meddr dansk
naturh. Foren. 139: 337-396.
Andersen NM. 1977. Fine structure of the body hair layers and morphology of the
spiracles of semiaquatic bugs (Insect, Hemiptera, Gerromorpha) in relation to
life on the water surface. Vidensk.
Meddr dansk naturh. Foren. 140: 7-37.
Andersen NM. 1982. The Semiaquatic Bugs (Hemiptera, Gerromorpha). Phylogeny,
adaptations, biogeography, and classification. Entomonograph 3: 1-455.
Andersen NM, Polhemus JT.
1976. 8. Water-striders (Hemiptera: Gerridae,
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Amsterdam: 187-224.
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skaters: differences in surface fine structures. Nature, Lond. 242: 132-133.
Cheng L. 1973b. Halobates. Ann. Rev.
Oceanogr. Mar. Biol. 11: 223-235.
Cheng L. 1974. Notes on the ecology of the
oceanic insect Halobates. Mar. Fish. Rev. 36: 1-7.
Cheng L. 1985. Biology of Halobates
(Heteroptera: Gerridae).
Ann. Rev. Ent. 30: 111-135.
Miyamoto S, Senta T. 1960. Distribution,
marine condition and other biological notes of marine water-striders, Halobates
spp., in the south-western sea area of Kyushu and western area of Japan Sea.
Sieboldia 2: 171-186.