Holoxantha Semenov, 1894

Plata-Negrache, Paulino & Alonso-Zarazaga, Miguel A., 2006, Nacerdochroa concolor (Brullé, 1839) (Coleoptera: Oedemeridae): taxonomy, ecology, life cycle and larval instars, Zootaxa 1373, pp. 37-52 : 41-49

Holoxantha Semenov, 1894 , a new synonym of Nacerdochroa Reitter, 1893

The following specimens were examined:

Nacerdochroa concolor . Several specimens from Canary Islands (El Hierro, Gran Canaria, La Gomera, La Palma, Tenerife).

Nacerdochroa caspica . 1 ɗ, 3 ΨΨ from Turkmenistan (St. Ajcha-Kujma; Molla-Kara, Bl. Desvela; Krasnovodsk.). 1 Ψ from Uzbekistan (St. Kara-Kul' Buj.). 1 ɗ from Azerbaijan (Baku). All specimens housed in the Museum of Saint Petersburg.

Nacerdochroa carinatopyga . 1 ɗ from Somalia (Scusciuban, Migiurtinia, Valle d. Darror) (Museo Civico di Storia Natural di Trieste).

The male genitalia of Nacerdochroa species is quite similar, i.e., penis without apical hooks, parameres very long and slender, ninth sternite without bifid medial projection, eighth sternite concealed and membranous, last visible sternite not emarginate. Both female genitalia and external features are also very similar.

Š vihla (1986) based the distinction of the subgenera Nacerdochroa and Holoxantha in the relative length of the last segment of posterior tarsi and the shape of the parameres, but such characters are not worth of more than a specific rank.

We then propose formally the following synonymy: Nacerdochroa Reitter, 1893 = Holoxantha Semenov, 1894 , syn. nov.

We are considering that there is a single genus, Nacerdochroa , whose representatives show a disjunct distribution, a fact not uncommon in some Canarian taxa, e.g. the genus Cephaloncus Westwood ( Melyridae , Malachiinae ). Even in Alloxantha , a Saharan species has been described later ( Alloxantha svihlai Vảzquez, 1993a), filling somewhat the gap with the species of the Near East (Vảzquez, 1993b).

Nacerdochroa concolor (Brull é, 1839)

Ditylus concolor Brullė, 1839: 70

Holoxantha concolor (Brullė) : Semenov, 1894: 472

Alloxantha hierrensis Franz, 1985: 102 : Ś vihla, 1988: 378 (synonymy) Nacerdochroa (Holoxantha) concolor (Brullė) : Ś vihla, 1986: 192

Distribution: Endemic to the Canary Islands, recorded previously from the islands of Tenerife, El Hierro, La Gomera, Gran Canaria and La Palma ( Machado & OromÌ, 2000); we confirm its presence in the last two islands.

Preimaginal states and life cycle are poorly known. Only Kubisz & Borowski (1999) gave fragmentary data on N. concolor , since they found larvae and pupae of this species in partially rotten roots of Euphorbia regis-jubae in the island of La Gomera; neither any of the preimaginal states nor the life cycle was fully described.

We describe here the different ontogenetic stages of N. concolor .

Description of the imago

Male: Length: 10–18 mm.

Elongate, subcylindrical, testaceous yellow, most of the dorsal surface uniformly covered with a very fine, short, appressed pubescence of golden-yellow colour, less uniform on ventral side (fig. 8). Head somewhat prominent, oblong, elongate, narrower (with eyes) than prothorax, covered with dense punctures of the approximate size of an ommatidium. Frons wide, flat, with two clear protuberances at antennal insertions, space between these with a slightly depressed area. Eyes rather large, transverse, reniform, slightly convex, placed below the antennal insertion in side view, fore margin slightly sinuate, separated by a distance less than the interantennal one. Ommatidia coarse. Clypeofrontal suture evanescent. Clypeus medium-sized, moderately transverse, sides strongly convergent towards truncate apical margin, outer angles obtuse, medially sulcate. Epistome yellow-ochreous. Genae clearly parallel, no visible neck. Vertex slightly convex. Labrum transverse, moderate in size, basal margin narrower than apical, the latter densely covered with very long yellow setae. Mandibles large, robust, rather elongate, yellowish, tinged with dark red at apical third (in some specimens black), with yellow setae at base, apex bifid by the presence of a deep subangular fissure forming two clear teeth. Maxillae with abundant yellowish pubescence on galea and lacinia; maxillary palps 4-segmented, 1st segment small, retracted, last long, rather dilated, obliquely triangular, knife-shaped, apical angle acute, front margin slightly curved. Labial complex with sclerotized postmentum and membranous prementum, this bearing 3-segmented labial palps; paraglossae and hypopharynx very pubescent. Antennae slender, simple, filiform, 11- segmented, inserted in front of eyes, moderately long, reaching, when folded back, slightly more than half the elytral length; antennomeres rather slender and moderately elongate, yellowish, with a crown of apical yellowish setae: 1st antennomere oblong, subclavate; 2nd obconical, about as long as half the 1st; 3rd similar to 2nd but twice as long; 4th with shape and proportions of 3rd, remaining antennomeres up to 10th, little longer, 11th slender and divided by a notch into two neat parts, a little shorter than 10th. Prothorax narrow, transverse, its width about 1.25 x its length; fore margin almost straight and wider than basal margin; anterior angles rounded, sides widest in fore third, narrowing gradually towards base; basal margin with a fine rim, more or less transversely impressed from the basal angles to the middle, margin at middle arched, obscuring base of scutellum. Dorsal surface slightly convex, densely punctured, punctures coarser than those on head, two pairs of transverse impressions near the fore and basal margins. In ventral view, without special characters, as in the general pattern of the family. Scutellum wide, tongue-shaped, apex rounded, medially slightly sulcate. Elytra dull, slightly convex, yellowish, more or less depressed along sutural margin behind scutellum, with dense, thin, regular, adpressed pubescence masking the almost unnoticeable, finely subrugose punctures. Humeri rounded but rather prominent, elytra widest at this level, 1.5 times as wide as pronotal width, narrowing from humeri backwards and then gradually widening upto two thirds of length, each elytron angularly curved separately from here to apex. Surface of each elytron with two slight longitudinal ribs, usually very thin and hardly visible. Elytra dehiscent from about midlength onwards, exposing wings (fig. 8). Legs slender, elongate, testaceous yellow, tarsal formula 5-5-4. Fore and middle coxae contiguous, conical, prominent; hind coxae separate, transverse. Fore legs with femora gradually widening from base to apex and as long as protibia; this clearly narrower and with two thin apical laterointernal spurs. Protarsi (fig. 12) 5-segmented, the 1st largest, obconical, as long as 2nd -4th together, 2nd obconical, slightly larger than 3rd, 3rd cordiform and as long as 4th, this also cordiform but slightly narrower and with ventral pads, 5th inserted in a dorsobasal excavation of 4th, elongate, longer than 4th, clavate. Mid legs longer but similar to fore legs; mesotarsi (fig. 13) also 5-segmented, the largest 1st subcylindrical, as long as as remainder together; 2nd subtriangular, much wider at apex and slightly longer than 3rd, similarly shaped, 4th cordiform, with ventral pads, 5th similar to that of protarsus but longer. Hind legs still longer, 1st metatarsomere largest (fig. 14), longer than remaining three, 2nd twice as long as third and rather subtriangular, 3rd smallest, cordiform, with ventral pads; last similar to those of pro- and mesotarsus, but longer. All last segments of tarsi with 2 simple claws. All tarsomeres except penultimate with simple pubescence, more visible on sides. Functional metathoracic wings present (fig. 17), heteromeroid venation reduced. Radial cell well developed, radial sector and anal cell present, median vein united to cubital. Abdomen with 5 visible sternites (3rd -7th) of the cryptogastric type ( Jeannel & Paulian, 1944). Eighth segment concealed in both sexes, modified in male, annexed to ovipositor and emarginate in female. Male genitalia with 7th segment modified as an outer envelope of the remainder of the apparatus, with a deeply notched sternite projecting 2 slender, acuminate lobes.

Eighth tergite always invaginated and reduced, formed by two lateral sclerotised plates joined by membrane. Ninth (genital) segment basically formed by two rods developing a sclerotised apical plate; its tergite, much reduced and membranous, with longitudinal reinforcement rods. Tegmenite (sclerotised part of membranous tube connecting genital segment and basal piece of tegmen) formed by two fused pieces. Tegmen (fig. 15 b) without differentiated basal piece (Š vihla, 1986), phallobase and parameres completely fused, no separating suture present, in a dorsal position in relation to penis (fig. 15 a), although in situ the entire structure can rotate, composed of a concave plate lodging the penis, with two strong laterobasal apodemes. Parameres well developed. Penis (fig. 16) consisting of a subcylindrical, well sclerotised tube, ejaculatory duct entering ventrobasally, internal sac without sclerotisations.

Female: Length: 15 –20 mm.

Very similar to male, from which it is difficult to distinguish. Usually larger and more robust, but positive sex identification requires the attentive observation of the abdominal apex (fig. 9), and if possible, dissection of the genitalia. When gravid, they are easier to distinguish because of the accentuated distention of abdominal segments full of eggs.

Eighth (genital) segment a tubular, membranous sheath with four reinforcement rods (two tergal and two sternal), pubescent at apex, a long, slender sternal apodeme without direct contact with these rods. Ovipositor (fig. 18) with long slender ventral baculi and short oblique baculi, coxites weakly sclerotised, with some setae; styli subapical, membranous and pubescent; proctiger well differentiated, its baculi separate. Its general structure is also very similar to that of the females of Alloxantha , thus showing no useful features for generic or specific separation ot taxa.

The rest of the genitalia are similarly rather uniform, mainly at the genus level. There is an elongate vagina with chitinous denticles, ending in the oviduct; a bursa copulatrix connecting with the vagina through a seminal channel placed in lateroapical position and two independent accessory glands opening directly into the bursa copulatrix.

The female genitalia are not well known in Oedemeridae . Hudson (1975) studied these in depth when treating the New Zealand representatives (we have used this author’s nomenclature for structures), and Vảzquez (1993a, 1996) studied some European and African species, respectively. Although internal female reproductive structures are poorly known across the Oedemeridae , it is very likely that they will prove important for classification.

Description of egg

The egg is elongate, almost cylindrical, with both ends rounded, whitish when laid and becoming yellowish when approaching eclosion. The size is usually over 1 mm long; the mean of 54 measured eggs is 1.2 mm.

Description of larvae

In general, the larvae of Oedemeridae are poorly known; Rozen (1958, 1959, 1960) is the only author that has paid detailed attention to the larvae of several, mainly Nearctic species. As a result, the following description is based on his studies.

Larvae elongate, cylindrical, weakly sclerotised (fig. 6). The head capsule slightly asymmetrical (fig. 19), particularly in ventral view (fig. 20). The gula is well differentiated but very short, and there is a prominent prehypopharynx (fig. 22 a) (“columnar projection”, sensu Rozen, 1958, 1959, 1960, exclusive of the Oedemeridae ), bearing small setae. This peculiar structure of prehypopharynx is exclusive of the Oedemeridae . The hypopharynx is of the normal type, inferiorly dentate and without pubescence; the maxillae have lacinia and galea fused, forming a rounded mala (fig. 21 a). The mandibles (fig. 23) are very characteristic, since, depending upon the viewpoint, they show a different number of teeth; in fact they have five and a well formed mola. On each segment, the thorax has small dorsal asperate ampullae and one pair of 5-segmented legs, each bearing one simple claw (fig. 24). The abdomen has no particular structures; the spiracles are of the annular type, urogomphi absent, apex of abdomen rounded (fig. 24).

Pupa

The pupa (fig. 7), as is usual in Tenebrionoidea and especially in Oedemeridae , does not show any feature allowing distinction at the species, genus or tribal level even at the subfamilial one, thence we do not attempt a description.

Life cycle

Imagines are strictly crepuscular to nocturnal. This is a feature that, according to Š vihla (1986) and Vảzquez (1993a) represents the primitive character of diverse evolutionary lines within Oedemeridae . The Oedemeridae that show this kind of behaviour are yellowish, testaceous to brownish in colour, while those having diurnal habits are more brightly coloured.

Because of their crepuscular to nocturnal activity, they have large eyes and are attracted to electromagnetic radiation sources in different frequencies of the spectrum, mainly UV, which can be used for control captures. They usually frequent trees or shrubs with aromatic flowers, probably guided by chemical stimuli, and feed on them, consuming principally nectar and pollen.

During the day adults shelter in protective places, being occasionally captured under stones, a fact already observed by Wollaston (1864). We have found them also under logs, cardboard and vegetal refuse. That is why they are difficult to capture and the main reason for poor representation in collections.

We did not observe courtship and copulation, but it seems likely that both occurred nocturnally. The fertilized females look for appropriate places for oviposition. These places must fulfil several essential requirements: darkness, high relative humidity (80–97 %) and a moderate woody-fibrose consistency. The trap system we used did not allow us to discern peak hours of activity. As for the dragon-tree, it must be relatively old to have acquired a woody consistency, since all attempts we have carried out for gravid females to oviposit on small dragon-trees have been unsucessful. This observation is in agreement with similar habits in other Oedemeridae (Vảzquez, 1993a, 2002b).

The eggs are laid separately, although they can be restricted to a small area, if it presents the required characteristics.

The first instar larvae measured 1.1–1.3 mm in length, and were negatively phototropic and positively thigmotropic, starting boring the substratum to form a gallery. In our samples, we have been able to observe that the first instar larvae possess mandibles not strong enough to cut healthy wood, this being the reason for the requirement of a high water content to help in mastication. If the substratum consistency is still more impaired by fungal or bacterial action, they can penetrate the decayed wood more efficiently..

Once the boring is started, the larvae form a gallery, usually in an obliquely upward direction. Since the starting hole is very small, it can easily go unnoticed. The deposition of faeces occlude the galleries, which can be relatively deep, since in our sampling it was necessary to dig out ca. 10 cm to find the larvae. The gallery becomes gradually broader as the larva grows.

We have deduced the existence of at least five larval instars before pupation, although there is some uncertainty in these data. The last larval instar, when the larva is about to pupate, reaches a length of 25 mm. We have collected larvae of different sizes (from neonate to terminal) belonging apparently to five different cephalic width classes, implying intermediate instars.

The time duration of the larval part of the life cycle could not be determined exactly in situ, but an extrapolation of the data known for other Oedemeridae showing the same habits allow us to consider a time span of 2–3 years. This hypothesis is strongly supported by data in Horion (1956), Liebenow (1979) and Crowson (1981) for other genera with similar behaviour ( Calopus, Ischnomera, Oedemera, Chrysanthia , etc.) of boring galleries in Quercus, Ulmus, Salix, Acer, Fagus, Castanea , etc. These authors also assert that pupation takes place inside the gallery, a fact we can confirm for N. concolor , and we have also found that it is not rare that the adult hibernates in this place for several months. Since the imago is supplied with strong mandibles, it is able to emerge from the gallery at an opportune moment.

Using gravid females obtained with UV light traps, we succeeded in getting eggs laid on roots of Euphorbia canariensis . The roots were kept at a constant laboratory temperature of 18ºC, with a variable but hardly oscillating ambient humidity. From the oviposition to the emergence of 5 adults 28 months elapsed (year 2003). No larvae were extracted during this experiment to avoid any interference in the normal development of their life cycle, but we could determine the length of the galleries they constructed, varying from 28 cm (the shortest one) up to 42 cm (the longest one). There is a direct relation between the length of the gallery and the size of the imago: the longer the gallery, the larger the adult.

Using the data we have collected and those obtained from the labels of specimens housed in different collections, we are convinced that there are two periods when the adults appear en masse: the beginning of spring and the end of summer. It is very rare to capture any specimen between mid autumn and the end of winter. We can also deduce from these data that the imaginal life span is very short, not surpassing two months. This fact is confirmed for similar species by other authors. We can also deduce that some generational overlap takes place, since at least twice we have captured simultaneously larvae, pupae and adults.