Morphology of the larvae of three Central European Strophosoma Billberg , 1820 ( Coleoptera , Curculionidae , Entiminae ) species

The larvae of Strophosoma (Strophosoma) capitatum (DeGeer, 1775), S. (Strophosoma) melanogrammum (Forster, 1771) and S. (Neliocarus) sus Stephens, 1831, are illustrated and re-described or described for the first time. The first larval instar, and the mature or an older larval instar, are illustrated, and a general description of the Strophosoma larva is given. The biological data obtained from breeding and field-collecting are compared and discussed in relation to the known life-cycle data.


Introduction
The short-nosed weevil genus Strophosoma is represented in Southwest Europe (mainly Spain) and Northwest Africa (Morocco, Algeria) by more than 40 species (Pelletier 2013).Only a few species have a widespread distribution and are present in Southwestern Asia (two species) or Central Europe (five species), among them the parthenogenetic Strophosoma melanogrammum (Forster, 1771).Van Emden (1952) characterized the larvae of four species; in three, the description was based on L 1 larvae.Only in S. faber (Herbst, 1785) were higher stages also taken into account.This species was previously described by Urban (1913).Little additional information was added by Scherf (1964) on Strophosoma larvae.He included some brief information on the morphology of S. melanogrammum, but without any drawings.Only Willis (1964) characterized the mature larva of S. melanogrammum in any detail, but this source, a thesis of the Queen's University Belfast, is not readily available.
In this paper the L 1 larvae of S. capitatum (DeGeer, 1775) and S. melanogrammum and the mature larva of S. melanogrammum are re-described, the mature larva of S. capitatum is described for the first time, and the young and an old larval instar of Strophosoma sus Stephens, 1831(syn. S. laterale (Paykull, 1792)) are also described for the first time.Priority is given to the chaetotaxy of the described larvae.

Materials and methods
The older larvae on which these descriptions are based on, were collected in the field or -in a few cases -bred in flower-pots.L 1 larvae were obtained from adults kept in the laboratory.Larvae of the polyphagous species Strophosoma capitatum and S. melanogrammum were dug out of soil under possible host plants or, in the case of the monophagous S. sus, under Calluna vulgaris (L.) Hull, and fixed in 80% ethanol.The specimens were examined under an optical stereomicroscope (Olympus SZ 60).Measurements were made by using calibrated oculars; first larval instars and older larvae of each species were measured using a calibrated microscopic eyepiece: The measurements (body length (BL), body height (BH), width and length of the head capsule (HW, HL)).Measurements of all the species described are summarized in Table 1.
Old larvae (2 exx.): 1 ex., 25.07.2012,collected from under roots of Calluna vulgaris in Berkhof (Fig. 44); 1 ex.and 1 immature adult, 02.09.2012, from breeding in a flower-pot with Calluna vulgaris in Hannover, Curculio Institute (Fig. 43).In these cases it is not known, whether the larvae were in the last or the penultimate instar.

Key to mature larvae of selected Strophosoma subgenera and species
The larva of Strophosoma (Neliocarus) faber distinguished according to Van Emden (1952) and Scherf (1964), the remaining species as in the present work.In S. sus based on the last or penultimate instar.

Remarks on the development of Strophosoma species
The data about the development of Strophosoma melanogrammum and S. capitatum (in part) have been mainly worked out and summarized by Grimm (1973) and Schauermann (1973).According to these sources both species develop in the Solling Mountains in the southern part of Niedersachsen (Germany) very similarly, and over a period of more than one year.After overwintering adults start to feed and to lay eggs from May to first half of July.Dieckmann (1980) found females with eggs still in the abdomen in August.The eggs are usually laid in clusters in any kind of sheltered situations, e.g. in cracks of cork, between dry leaves of litter, in soil (probably in crevices), between leaf or flower buds of the host plant, on the ground in needles of pine shoots (Breese 1948;Grimm 1973) and, in the laboratory, between rolled or pleated absorbent paper or between paper and the substrate.The eggs are held together by a viscid fluid.This agrees with observations in the laboratory, documented by Sprick and Stüben (2012), who found bulks of eggs deposited between the layers of absorbent paper close to its edges, where these were laid by the female's ovipositor, and by the observations of Wolcott (1933) on neotropical Entiminae genera of the (same) former subfamily Brachyderinae.He had fixed double paper strips in the field on the top of stakes beneath cultivated young Citrus plants, and found many eggs deposited between the paper strips or sheets.Larvae then develop in soil and feed mainly on fine roots until the third or last (fourth) instar and overwinter.In the following year the larvae complete their development and pupate, mainly in August.Adults of the new generation emerge from September to November and single specimens also in spring of the succeeding year.Adults climb for maturation feeding in tree crowns in spring, late summer and early autumn.They also feed in the herb layer and in leaf litter.Urban (1913) gave a strikingly different time of pupation for S. faber: the second half of May, producing a newly emerged weevil in June.
According to Schauermann (1973) S. melanogrammum and S. capitatum have 4 larval instars.But there is no information about how these instars were differentiated.The method of Sprick and Gosik (2014) for the determination of larval instars failed in these species, as the eyes are strongly protruding over the edge of the head in the pupae.
The breeding of Strophosoma melanogrammum and S. capitatum in the climate chamber, which was very successful in the case of Otiorhynchus larvae (Gosik et al. 2016), did not result in any S. capitatum larvae (two attempts) and only one larva of S. melanogrammum (four attempts), although host plants, the size of flower-pots and the soil substrate were varied.Also an addition of eggs bred before in keeping boxes was not successful.For these reasons no pupae could be obtained by breeding.Field-collecting was conducted only rarely in August, the main month of pupation.
The larval development of any other Strophosoma species is apparently unknown.Breeding of Strophosoma sus in two flower-pots with Calluna vulgaris and Erica arborea L. revealed the following data (but only in the pot with Calluna): egg-laying started in May (first adults being already active in April) and a newly emerged, weak, adult and a larva were obtained in the beginning of September; four further larvae were seen in the flower-pots, but a further check 3 or 4 weeks later did not reveal any larva, pupa or adult weevil.The reasons for the failure of the continuation of this breeding attempt are not known.Three pupae of Strophosoma cf.sus were found in the field at the heathland site near Berkhof with Calluna vulgaris and young Pinus sylvestris trees, where S. sus, S. capitatum and, rarely, also S. fulvicorne (Walton, 1846) were present (Gosik and Sprick 2013).Two pupae were found on 9 th July and one on 2 nd September, indicating an earlier start of pupation than in S. capitatum or S. melanogrammum from the Solling Mountains.This is not surprising, as the heathland near Berkhof with a population of S. sus is a warm and dry lowland area that allows earlier development.From this, it would be of interest to determine whether the development of S. capitatum or S. melanogrammum is also possible within one year at such warm sites, or whether larval overwintering and diapause are obligatory in these species.

General remarks about the morphology of Strophosoma larvae
Some basic information regarding the morphology of the larva of Strophosoma (Neliocarus) faber (Herbst, 1785), with drawings of head, mouthparts and apex of abdomen, can be found in the work of Urban (1913).However, the main sources of information about morphology (and egg-laying habits) of Strophosoma larvae are studies of Van Emden (1950, 1952), containing descriptions of the first larval instar of S. melanogrammum, S. capitatum and S. (Neliocarus) nebulosum Stephens, 1831 (syn.S. retusum Marsham, 1802), as well as of the mature S. faber larva.Van Emden (1952) also specified a number of characters of taxonomic importance for the entire genus Strophosoma, such as the presence of 3 or 4 pds on Abd.VIII, the conspicuously sclerotized apex of the abdomen, the shape of the premental sclerite, the proportions of setae on the pedal lobes, and the pds proportions on Abd.VIII.
The presence of four ventral malar setae in larvae of the genus Strophosoma confirms the observations of Marvaldi (1998a) that this character, as well as the shape of the antennal sensorium (Marvaldi 1997(Marvaldi , 2003)), can be considered an additional apomorphic character for larvae of the subfamily Entiminae, while other Curculionidae, e.g.Cyclominae in the sense of Marvaldi (1998bMarvaldi ( , 2003)), possess the plesiomorphic number of five vms.
Further information on the morphology of preimaginal stages of the genus Strophosoma was provided by Willis (1964), who presented a very detailed, but unfortunately sparsely illustrated, description of the mature larva of S. melanogrammum.In addition to valuable information on the biology of reproduction and the morphology of larval stages of selected Entiminae species (genera Otiorhynchus Germar, 1822, Barynotus Germar, 1817, Sciaphilus Schönherr, 1823 and Strophosoma), Willis (1964) described a few general patterns valid for the morphology of weevils.Moreover, he observed changes in the proportions and relative lengths of setae when the larvae entered successive developmental stages.For this reason the key to the genus Strophosoma of Van Emden (1952), enabling identification of first-instar larvae to species level based on the proportions of setae, should be regarded -according to Willis (1964) -as at least inadequate for an identification of older larval instars.Changes in the proportions and shape of setae of weevil larvae when passing through successive developmental stages were also observed in later research on Entiminae (Gosik and Sprick 2012) and Hyperinae larvae (Skuhrovec 2004(Skuhrovec , 2006(Skuhrovec , 2007)).Van Emden (1952) gave differences in the relative lengths of setae on pedal areas as the most important difference between Strophosoma sensu stricto and Neliocarus, but he did not mention the presence of four pds on the 8 th abdominal segment in Neliocarus versus three pds in Strophosoma.
The differences in the number of setae and in the morphology of abdominal segments which are observed in the species treated in this paper (S.capitatum, S. melanogrammum and S. sus) as well as in the previously described larva of S. faber (Van Emden 1952;Scherf 1964) show that Strophosoma species can be easily recognized in both mature stages and in younger larvae.The differences between them (e g. number of setae and presence or absence of asperities on the epipharynx) seem to sup-port the existing division into subgenera (Strophosoma and Neliocarus).However, due to a lack of information about the morphology of the larvae of the remaining subgenera (Morphostrophus Flach, 1907, Pelletierius Alonso-Zarazaga & Lyal, 1999and Subcaulostrophus Flach, 1907according to Pelletier (2013)), a definitive evaluation of these characters will only be possible after further research.

th abdominal segment: adaptation to the environment versus phylogenetic relationship
Because of the morphology of the eighth abdominal segment, especially the sclerotization and the dark colour, larvae of the genus Strophosoma are among the most easily recognized weevil larvae.Only the larvae of the genera Philopedon Schönherr, 1826, and Tanymecus Germar, 1817, are similar to Strophosoma larvae with respect to the morphology of this segment ( Van Emden 1950).This leads to the question: are these genera more closely related to each other than to other Entiminae genera?Here we can state the following: Smreczyński (1966) placed the genus Strophosoma in a separate tribe, Strophosomini, in a close relationship with the tribe Cneorhinini Lacordaire, 1863, with the genus Philopedon, whereas Tanymecus was placed in another subfamily (Tanymecinae).Dieckmann (1980) did not use tribes, but he also placed Philopedon next to Strophosoma (and between them only Cneorhinus Schönherr, 1823 with unknown larvae).Tanymecus was again placed in another subfamily.Today all these genera are members of the subfamily Entiminae, but they are placed in three different tribes: Brachyderini with Strophosoma (Pelletier 2013), Cneorhinini with Philopedon (Alonso-Zarazaga 2013) and Tanymecini with Tanymecus (Li Ren et al. 2013).A carefully reconstructed phylogeny, combining molecular, morphological and biological data, is still lacking.
Even if we do not know anything about the function of this conspicuous abdominal structure, the possibility that this is an adaptation to the environment has to be taken into account.It is found in a few genera of Entiminae only, larvae of which develop in soil.But there is no information as to whether these larvae exhibit behaviour or habits that are different from those larvae without a sclerotized 8 th abdominal segment.

Figures 40- 45 .
Figures 40-45.Sampling sites, host plants, larvae, teneral and mature adults.40 -sampling site of Strophosoma capitatum in a beech forest in the Deister Mountains southwest of Hannover, 41 -adult S. capitatum feeding on Salix caprea in a pine forest on the outskirts of Celle (Niedersachsen), 42 -habitat of S. melanogrammum near Ilsenburg (Sachsen-Anhalt) in the National Park Harz, a broken down spruce plantation, now containing a pioneer forest with young birch trees, 43 -mature larva and fresh adult of S. sus from breeding, 44 -searching site for immature stages of S. sus between the roots of Calluna vulgaris in the southern part of the Lower Saxonian heathland near Berkhof, 45 -mature larvae of S. melanogrammum found between the roots of Cytisus scoparius near Brelingen in the north of Hannover.

Table 1 .
Measurements of characteristic body parts of the Strophosoma species studied.HW -head width; HW* head width with prominent eyes included; HL -head length; BL -body length; BH -body height; L1 -first instar larvae, LM -larvae of last instars.All measurements in millimeters[mm].