Box 55 Eggtending fathers the giant water bugs

Care of eggs by adult insects is common in those that show sociality (Chapter 12), but tending solely by male insects is very unusual. This behavior is known best in the giant water bugs, the Nepoidea, comprising the families Belosto-matidae and Nepidae whose common names -giant water bugs, water scorpions, toe biters - reflect their size and behaviors. These are predators, amongst which the largest species specialize in vertebrate prey such as tadpoles and small fish, which they capture with raptorial fore legs and piercing mouthparts. Evolutionary attainment of the large adult body size necessary for feeding on these large items is inhibited by the fixed number of five nymphal instars in Heteroptera and the limited size increase at each molt (Dyar's rule; section 6.9.1). These phyloge-netic (evolutionarily inherited) constraints have been overcome in intriguing ways: by the commencement of development at a large size via oviposition of large eggs, and, in one family, with specialized paternal protection of the eggs.

Egg tending in the subfamily Belostomatinae involves the males "back-brooding" - carrying the eggs on their backs - in a behavior shared by over a hundred species in five genera. The male mates repeatedly with a female, perhaps up to a hundred times, thus guaranteeing that the eggs she deposits on his back are his alone, which encourages his subsequent tending behavior. Active male-tending behavior, called "brood-pumping", involves underwater undulating "press-ups" by the anchored male, creating water currents across the eggs. This is an identical, but slowed-down, form of the pumping display used in courtship. Males of other taxa "surface-brood", with the back (and thus eggs) held horizontally at the water surface such that the interstices of the eggs are wet and the apices aerial. This position, which is unique to brooding males, exposes the males to higher levels of predation. A third behavior, "brood-stroking", involves the submerged male sweeping and circulating water over the egg pad. Tending by the male results in more than 95% successful emergence, in contrast to death of all eggs if removed from the male, whether aerial or submerged.

Members of the Lethocerinae, sister group to the Belostomatinae, show related behaviors that help us to understand the origins of aspects of these paternal egg defenses. Following courtship that involves display pumping as in Belostomatinae, the pair copulate frequently between bouts of laying in which eggs are placed on a stem or other projection above the surface of a pond or lake. After completion of egg-laying, the female leaves the male to attend the eggs and she swims away and plays no further role. The "emergent brooding" male tends the aerial eggs for the few days to a week until they hatch. His roles include periodically submerging himself to absorb and drink water that he regurgitates over the eggs, shielding the eggs, and display posturing against airborne threats. Unattended eggs die from desiccation; those immersed by rising water are abandoned and drown.

Insect eggs have a well-developed chorion that enables gas exchange between the external environment and the developing embryo (see section 5.8). The problem with a large egg relative to a smaller one is that the surface area increase of the sphere is much less than the increase in volume. Because oxygen is scarce in water and diffuses much more slowly than in air (section 10.3) the increased sized egg hits a limit of the ability for oxygen diffusion from water to egg. For such an egg in a terrestrial environment gas exchange is easy, but desiccation through loss of water becomes an issue. Although terrestrial insects use waxes around the chorion to avoid desiccation, the long aquatic history of the Nepoidea means that any such a mechanism has been lost and is unavailable, providing another example of phylogenetic inertia.

In the phylogeny of Nepoidea (shown opposite in reduced form from Smith 1997) a stepwise pattern of acquisition of paternal care can be seen. In the sister family to Belostomatidae, the Nepidae (the waterscorpions), all eggs, including the largest, develop immersed. Gas exchange is facilitated by expansion of the chorion surface area into either a crown or two long horns: the eggs never are brooded. No such chorionic elaboration evolved in Belostomatidae: the requirement by large eggs for oxygen with the need to avoid drowning or desiccation could have been fulfilled by oviposition on a wave-swept rock, although this strategy is unknown in any extant taxa. Two alternatives developed: avoidance of submersion and drowning by egg-laying on emergent structures (Lethocerinae), or, perhaps in the absence of any other suitable substrate, egg-laying onto the back of the attendant mate (Belostomatinae). In Lethocerinae, watering behaviors of the males counter the desiccation problems encountered during emergent brooding of aerial eggs; in Belostomatinae, the pre-existing male courtship pumping behavior is a pre-adaptation for the oxygenating movements of the back-brooding male. Surface-brooding and brood-stroking are seen as more derived male-tending behaviors.

The traits of large eggs and male brooding behavior appeared together, and the traits of large eggs and egg respiratory horns also appeared together, because the first was impossible without the second. Thus, large body size in Nepoidea must have evolved twice. Paternal care and egg respiratory horns are different adaptations that facilitate gas exchange and thus survival of large eggs.

Hymenoptera eject the eggs from the opening of the genital chamber at the base of the modified ovipositor. However, in most wasps the eggs pass down the canal of the ovipositor shaft, even if the shaft is very narrow (Fig. 5.11). In some parasitic wasps with very slender ovipositors the eggs are extremely compressed and stretched as they move through the narrow canal of the shaft.

The valves of an insect ovipositor usually are held together by interlocking tongue-and-groove joints, which prevent lateral movement but allow the valves to slide back and forth on one another. Such movement, and sometimes also the presence of serrations on the tip of the ovipositor, is responsible for the piercing action of the ovipositor into an egg-laying site. Movement of eggs down the ovipositor tube is possible because of many posteriorly directed "scales" (microsculpturing) located on the inside surface of the valves. Ovipositor scales vary in shape (from plate like to spine-like) and in arrangement among insect groups, and are seen best under the scanning electron microscope.

The scales found in the conspicuous ovipositors of crickets and katydids exemplify these variations (Orthoptera: Gryllidae and Tettigoniidae). The ovipositor of the field cricket Teleogryllus commodus (Fig. 5.12) possesses overlapping plate-like scales and scattered, short sensilla along the length of the egg canal. These sensilla may provide information on the position of the egg as it moves down the canal, whereas a group of larger sensilla at the apex of each dorsal valve presumably signals that the egg has been expelled. In addition, in T. commodus and some other insects, there are scales on the outer surface of the ovipositor tip, which are orientated in the opposite direction to those on the inner surface. These are thought to assist with penetration of the substrate and holding the ovipositor in position during egg-laying.

Fig. 5.10 The generalized structure of a libelluloid dragonfly egg (Odonata: Corduliidae, Libellulidae). Libelluloid dragonflies oviposit into fresh water but always exophytically (i.e. outside of plant tissues). The endochorionic and exochorionic layers of the eggshell are separated by a distinct gap in some species. A gelatinous matrix may be present on the exochorion or as connecting strands between eggs. (After Trueman 1991.)

Fig. 5.10 The generalized structure of a libelluloid dragonfly egg (Odonata: Corduliidae, Libellulidae). Libelluloid dragonflies oviposit into fresh water but always exophytically (i.e. outside of plant tissues). The endochorionic and exochorionic layers of the eggshell are separated by a distinct gap in some species. A gelatinous matrix may be present on the exochorion or as connecting strands between eggs. (After Trueman 1991.)

Fig. 5.11 A female of the parasitic wasp Megarhyssa nortoni (Hymenoptera: Ichneumonidae) probing a pine log with her very long ovipositor in search of a larva of the sirex wood wasp, Sirex noctilio (Hymenoptera: Siricidae). If a larva is located, she stings and paralyses it before laying an egg on it.

Fig. 5.11 A female of the parasitic wasp Megarhyssa nortoni (Hymenoptera: Ichneumonidae) probing a pine log with her very long ovipositor in search of a larva of the sirex wood wasp, Sirex noctilio (Hymenoptera: Siricidae). If a larva is located, she stings and paralyses it before laying an egg on it.

group of sensilla group of sensilla

Fig. 5.12 Tip of the ovipositor of a female of the black field cricket, Teleogryllus commodus (Orthoptera: Gryllidae), split open to reveal the inside surface of the two halves of the ovipositor. Enlargements show: (a) posteriorly directed ovipositor scales; (b) distal group of sensilla. (After Austin & Browning 1981.)

Fig. 5.12 Tip of the ovipositor of a female of the black field cricket, Teleogryllus commodus (Orthoptera: Gryllidae), split open to reveal the inside surface of the two halves of the ovipositor. Enlargements show: (a) posteriorly directed ovipositor scales; (b) distal group of sensilla. (After Austin & Browning 1981.)

In addition to the eggshell, many eggs are provided with a proteinaceous secretion or cement which coats and fastens them to a substrate, such as a vertebrate hair in the case of sucking lice, or a plant surface in the case of many beetles (Fig. 5.9). Colleterial glands, accessory glands of the female reproductive tract, produce such secretions. In other insects, groups of thin-shelled eggs are enclosed in an ootheca, which protects the developing embryos from desiccation. The colleterial glands produce the tanned, purse-like ootheca of cockroaches (Taxobox 15) and the frothy ootheca of mantids, whereas the foamy ootheca that surrounds locust and other orthopteran eggs in the soil is formed from the accessory glands in conjunction with other parts of the reproductive tract.

Beekeeping for Beginners

Beekeeping for Beginners

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