Gryllacrididae (Orthoptera: Ensifera) in southern Africa.
Gryllacrididae (leaf-rolling or raspy crickets) are a cosmopolitan, though largely southern hemisphere (about one-third of the world's 600-odd species are known from Australia; Rentz 1997), family of Stenopelmatoidea with only a few representatives in southern Africa. But since the group is poorly studied there, having been last revised by Karny (1929), the fauna is quite likely to be significantly richer than assumed. Three genera with a total of ten species and subspecies have been recorded from the region.
Gryllacridids are robust, non-jumping crickets with stout, spiny legs. The exceptionally long antennae are rolled or curled around the body when the cricket is at rest (Figs 1-2). The southern African species are medium-sized (adults about 15 mm long), pale brown with soft bodies. Ametroides Karny, 1928 and Glomeremus Karny, 1937 (Figs 1-3) species are totally wingless while Stictogryllacris Karny, 1937 species have long tegmina and functional hind wings (Figs 4-5). Mature females have an ovipositor of roughly body-length (Figs 2, 5). In Stictogryllacris the tegmina are about twice the length of the abdomen and protrude a considerable distance beyond the tip of the body (Figs 4-5). As far as is known, all southern African species are arboreal and nocturnal and build shelters by spinning leaves together; elsewhere soil-burrowing species are known in which the silk is used to stabilize the surrounding soil (Morton and Rentz 1983). Gryllacridids produce often loud sounds (the origin of the common name "raspy crickets" coined by Rentz (1996), specifically for Australian species) either during defensive displays or while drumming on the substrate in intra-specific communication. Sound is produced by rubbing spines on abdominal tergites against tubercles on the inner surface of the hind legs (Field and Bailey 1997). All species lack tympana so sound communication is via surface vibration.
Silk-production has evolved at least 23 times in 17 orders of insects (Sutherland et al. 2010), sometimes multiple times in larger orders (twice in Neuroptera, twice in Coleoptera, three times in Diptera, and six times in Hymenoptera). Silk production has apparently evolved three times in Orthoptera: twice in different clades of Anostostomatidae--once in Lezina Walker, 1869 species, which occur in southwestern Asia and in northern and northeastern Africa, and once in Cnemotettix Caudell, 1916 of western North America (Vandergast et al. 2017)--and in Gryllacrididae where it is universal, and clearly monophyletic (Walker et al. 2012, Vandergast et al. 2017). Although silk is used during some stage of the life-cycle of all of these insect groups, in most it is produced only during a short-lived process or during only one life-stage. In only two orders, Embioptera and Orthoptera, however, is silk produced throughout the life-cycle of its members.
Members of the Gryllacrididae have certain unusual and unique characteristics, the foremost of which is the ability to produce silk, while another, recorded in some Glomeremus, is that they feed on nectar which is imbibed through a network of special maxillary microtrichiae that connect the maxilla and mandibles by capillary tubes. This adaptation essentially deviates from the typical biting and chewing mouthparts in Orthoptera to functionally one of sucking or fluid-feeding (Hugel et al. 2010, Krenn et al. 2016). Due to this adaptation to nectar-feeding, one species of Gryllacrididae from La Reunion in the Mascarene Islands, Glomeremus orchidophilus Hugel et al., 2010, evolved to become the only known orthopteran pollinator (Micheneau et al. 2010).
Silk is produced in one of three positions on the insect body: in the labium (as modified "salivary" glands), Malpighian tubules, or in a variety of dermal glands. The latter include silk-secreting accessory sex glands. Labial glands account for most examples of silk-production in insects such as the ubiquitous silk-spinning in Lepidoptera larvae. Dermal glands follow in terms of frequency of occurrence, while the production of silk by Malpighian tubules is rare (Sutherland et al. 2010). Examples of dermal gland production are found in Embioptera of all stages and ages in which the glands are situated in the prothoracic tarsomeres. The main function of the silk is for lining residence tunnels amongst debris and under bark where they live. Sexual accessory glands as are found in male Archaeognatha Borner, 1904 and Zygentoma Borner, 1904, function to spin silken threads that lead females to a spermatophore or silken mats on which spermatophores are deposited. The most familiar examples of Malpighian tubules producing silk are known from Neuroptera larvae, all of which spin pupal cocoons with silk.
Gryllacridids of both sexes are capable of spinning silk from soon after hatching until they die and the function is exclusively for construction of day-time shelters. These may be completed within 24 hours and are returned to repeatedly unless damaged, apparently by following pheromone trails. Crickets seal themselves into the shelters by closing the entrance with a silken flap through which they cut an access hole with the mandibles to enable them to emerge to forage. This is done repeatedly after every emergence and return to the shelter. Adjacent leaves may be pulled together and held with the tarsi while they are spun together (Fig. 6). The shelters are thought to function mainly in anti-predator defense, although in soil-frequenting species in arid regions, protection against desiccation is presumed (Walker et al. 2012). Most species are omnivorous although predation of sessile insects and spiders has been recorded (Hale and Rentz 2001).
The diameter of each silk strand produced increases with age --in the Australian species, Apotrechus illawarra Rentz, 1990, the diameter of a strand increases threefold between early and late instar crickets (Walker et al. 2012). Silk strands are produced by the labial glands from which a droplet of fluid issues and is formed when the labium is touched against the substrate then drawn away from the droplet (Figs 7-8). Single strands are spun from one substrate and attached to another. This is done repeatedly, resulting in thicker fibers, or individual strands are added together or crossed to form a film. Where fibers touch, they stick together, eventually forming a mat. Silk is added to the existing mats over time until the inside of the shelter is more or less covered in silk sheets (Walker et al. 2012; see Fig. 1).
In this short communication, we review the limited information available regarding southern African Gryllacrididae in the hopes that future researchers will be encouraged to study this elusive, but fascinating, group. We present a key and images to genera found in southern Africa, which will hopefully assist in future identifications of southern African Gryllacrididae.
Detailed photographs illustrating elusive Gryllacrididae behaviors were taken by H. de Klerk opportunistically from 1985-2017 during his hiking trips throughout southern Africa's natural areas using predominantly Nikon equipment. Most Gryllacrididae were encountered at night by carefully surveying surrounding vegetation until movement of antennae was observed and an individual was spotted. All images were taken in situ with either a 105 mm or a 200 mm Nikkor macro lens with flash illumination using multiple flashes.
Results and discussion
Diagnosis of southern African Gryllacrididae.--Gryllacrididae are most easily confused with Stenopelmatidae but do form a distinct monophyletic clade (Vandergast et al. 2017). Southern African Stenopelmatidae tend to be more robust in appearance and have shorter and thicker antennae than southern African Gryllacrididae. Gryllacridids are characterized by depressed and soft tarsi with prominent lateral lobes (Hale and Rentz 2001). Female stenopelmatids have reduced, flap-like ovipositors (see Weissman and Bazelet 2013, fig. 7), whereas southern African Gryllacrididae have long, sword-like ovipositors typical of Ensifera. Adult male Stenopelmatidae have lateral hooks on the anal plate (tenth tergite + epiproct, see Weissman and Bazelet 2013, fig. 6), while Gryllacrididae males have an enlarged ninth tergite at their abdominal apex (Gorochov 2001). All southern African Stenopelmatidae are obligatorily apterous, while Gryllacrididae can be either macropterous or apterous, depending on the species. Gryllacrididae have mouthparts specially adapted for silk production, which include well-developed maxillae and labial palps with specialized structures, while Stenopelmatidae have simple chewing mouthparts with well-developed, long, but unmodified labial palps. Both gryllacridids and stenopelmatids in southern Africa may have pegs interior to the hind femur used for femoro-abdominal stridulation, and both groups lack tympana on the fore tibiae. Furthermore, gryllacridids have heart-shaped heads when viewed head-on; stenopelmatids do not.
Ametroides (no images available) is an African genus with two of the 13 species found in southern Africa--the rest are restricted to central Africa. Males and females are totally without tegmina or hind wings.
Glomeremus (Figs 1-3) is the largest gryllacridid genus in Africa, with a total of 18 species spread across sub-Saharan Africa and some Mascarene Islands. Six species and subspecies are known from southern Africa. Both sexes lack both fore- and hind wings.
Stictogryllacris (Figs 4-8) has nine species, seven from central Africa and two from southern Africa. All species are fully-winged.
Key to the southern African genera of Gryllacrididae; adapted from Karny (1929) 1 Tegmina and wings fully developed Stictogryllacris Karny (Figs 4-5) - Tegmina and wings totally absent 2 2 Fore and middle tibiae with 3 to 4 spines on either side (the apical spines excepted) Glomeremus Karny (Figs 1-3) - Fore and middle tibiae with only 2 spines on either side (the apical spines excepted) Ametroides Karny
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CLARKE SCHOLTZ (1), CORINNA S. BAZELET (2), HENNIE DE KLERK (3)
(1) University of Pretoria, Department of Zoology and Entomology, Private Bag X20, Hatfield 0028, South Africa.
(2) Stellenbosch University, Department of Conservation Ecology and Entomology, Private Bag X1, Matieland 7602, South Africa.
(3) 14 Elgar Street SW5, Park South, Vanderbijlpark 1910, South Africa.
Corresponding author: Clarke Scholtz (email@example.com)
Academic editor: Juliana Chamorro-Rengifo | Received 10 September 2018 | Accepted 20 November 2018 | Published 10 December 2018
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|Title Annotation:||Short Communication|
|Author:||Scholtz, Clarke; Bazelet, Corinna S.; de Klerk, Hennie|
|Publication:||Journal of Orthoptera Research|
|Date:||Jul 1, 2018|
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