Measurements (in mm). Body length: 10.0–14.0 (mean 12.0). The widest place in the body (abdominal segments II–VI) measures up to 2.5. Head width: 0.9–1.1 (mean 1.0).

Colouration. Dark brown to black head (Fig. 7). All thoracic and abdominal segments greenish with white longitudinal stripes on both sides of body, but this larva also has a thick longitudinal yellow stripe and parallel longitudinal pink to violet stripes in its dorsal part with small black short stripes inside; all setae are thorn-like, located on distinct black protuberances in very thin white transversal lines (Figs 7–10, 16).

Vestiture. Body elongated, slightly curved, rounded in cross section (Fig. 7). Setae on body thin, different in length (short to relatively long), black thorn-like, located on distinct black protuberances.

Head capsule (Fig. 1). Head suboval, flattened laterally, endocarinal line absent. Frontal sutures on head distinct, extended to antennae. Two stemmata (st), in the form of a dark pigmented spot with convex cornea, both located on each side anterolaterally, close to each other. Des1 and des2 located in upper part of the central part of epicranium, des1 near to the middle part of epicranium, and des2 near to side of epicranium, des3 located anteriorly on epicranium near to frontal suture, des4 located in the central part of epicranium, des5 located anterolaterally; all des very long, des3 and des5 slightly longer than remaining three setae (Fig. 1). Fs1 and fs3 placed medially, fs2 absent, fs4 located anterolaterally, and fs5 located laterally, close to the epistoma; all setae long to very long, fs5 distinctly longer than the remaining four setae (Fig. 1). Les1–2 as long as des1; ves1–2 short. Epicranial area with four postepicranial setae (pes1–4) and two sensilla.

Antennae located at the end of the frontal suture on each side, membranous and slightly convex basal article bearing one conical triangular sensorium, relatively long; basal membranous article with three sensilla different in both shape and length (Fig. 5).

Clypeus (Fig. 2) approx. 3 times as wide as long with two relatively long cls, almost equal in length, localized posterolaterally, and one sensillum; anterior margin rounded to the inside; median part covered by thorn-shaped cuticular processes.

Mouth parts. Labrum (Fig. 2) approximately 3.2 times as wide as long, with three pairs of piliform lms, of different lengths; lms3 distinctly shorter than longer lms1 and lms2; lms1 placed close to the margin with clypeus, lms2 located anteromedially and lms3 located anterolaterally; anterior margin double sinuate. Epipharynx (Fig. 3) with three very short, piliform als, almost equal in length; one very short piliform ams; mes not distinct (but apparently there are two setal bases that may correspond to two small setae, not drawn); labral rods (lr) slightly elongated, sub-oval, apical part more sclerotised. Mandibles (Fig. 4) distinctly broad, trifid, tooth of unequal height; slightly truncate; both mds relatively long, piliform, located in distinct holes. Maxilla (Fig. 6) stipes with one stps, two pfs and one mbs, stps and pfs1–2 very long, pfs1 distinctly shorter than pfs2, mbs very short; mala with six bacilliform dms; five very short to minute vms, almost equal in length; vms distinctly shorter than dms. Maxillary palpi with two palpomeres; basal palpomere with one very short mxps and two sensilla; length ratio of basal and distal palpomeres: 1:0.8; distal palpomere with one sensillum and a group of conical, cuticular apical processes. Praelabium (Fig. 6) oval-shaped and distinctly elongated, with one relatively long prms; ligula with sinuate margin and two piliform very short to minute ligs, unequal in length; premental sclerite well visible. Labial palpi with two palpomeres; length ratio of basal and distal palpomeres: 1:0.9; distal palpomere with one sensillum and short, cuticular apical processes; basal palpomere with one dorsal sensillum. Postlabium (Fig. 6) with three pms, pms1 located anteriorly, remaining two pairs laterally; pms1 and pms3 relatively long, pms2 distinctly longer than others; surface of postlabium partly covered by cuticular processes.

Thorax. Prothorax distinctly smaller than meso- and metathorax. Spiracle bicameral, placed between the pro- and mesothorax (see Material and methods). Prothorax (Fig. 8) with 11 short to minute prns unequal in length, only 3 of them on small weakly pigmented dorsal sclerite, this sclerite subdivided in two triangular plates medially; two short ps and one short eus. Mesothorax (Fig. 8) with one short and one minute prs; four short pds; two short as; two very short ss; one short eps; one short and one minute ps; and one short and one minute eus. Chaetotaxy of metathorax (Fig. 8) almost identical to that of mesothorax. Each pedal area of thoracic segments well separated, with five short to minute pda, three of them on pigmented pedal area, unequal in length.

Abdomen. Abdominal segments I–VI of almost equal length, next abdominal segments decreasing gradually to the terminal parts of the body. Abdominal segment X reduced to four anal lobes of unequal size, the dorsal being distinctly the largest, the lateral pair equal in size, and the ventral lobe very small. Anus located terminally; ambulatory ampullae bilobate to circular. Spiracles bicameral, the eight abdominal spiracles located laterally, close to the anterior margin of abdominal segments I–VIII. Abdominal segments I–VII (Figs 9–10) with one short and one minute prs; five short pds; one short ss; two short to very short eps of almost equal length; one short and one minute ps; one short lsts; one short and one minute eus. Abdominal segment VIII (Fig. 10) with one very short and one minute prs; five very short pds, pds2 and pds4 not in line; one very short ss; two very short eps of almost equal length; one very short and one minute ps; one very short lsts; and one very short and one minute eus. Abdominal segment IX (Fig. 10) with four ds (two ds very short, two ds minute); two very short ps; and one very short and one minute sts. Abdominal segment X (Fig. 10) without setae.

Measurements (in mm). Body length: 7.0–8.0 (♂ 8.0; ♀ 7.0); at the widest region: 4.5–5.0. The widest place in the body is commonly between the apex of the meso- or metafemora. Unfortunately, both pupae were damaged and the measurements are not precise.

Colouration. Body yellowish with greenish abdomen (Figs 17, 19).

Morphology (Figs 11–13). Body stocky, cuticle smooth. Rostrum relatively long, approximately 2.5 times as long as wide, extended to metacoxae. Antennae relatively long and stout. Pronotum from 1.7 to 1.8 times as wide as long. Mesonotum and metanotum of almost equal length. Abdominal segments I–IV of almost equal length; abdominal segment V semi-circular, next abdominal segments diminish gradually to the end of the body. Abdominal segments VI–IX distinctly smaller than other abdominal segments. Gonotheca (abdominal segment IX) in females (one specimen) divided. Sexual dimorphism in weevil pupae is visible mainly in the length of rostrum and in the structure of abdominal segment IX: gonotheca of ♂ undivided and divided in ♀.

Chaetotaxy (Figs 11–13). Setae short to very short, unequal in length, light yellow, orange up to black. Setae well visible. Unfortunately, both pupae were damaged and it was not possible to observe chaetotaxy on some parts of body. Head capsule includes only three sos, one os and one pas. Rostrum with two rs. Setae on head capsule straight, as short as the remaining setae on thoracic and abdominal segments. Pronotum with two as, two ls and three pls, ds not observed. Dorsal parts of mesothorax with one seta located posteromedially, one seta posterolaterally and one seta located along its anterior margin. Dorsal parts of metathorax with one seta located along its anterior margin. Coxa with one very short seta (cs). Each apex of femora with groups of two fes. Abdominal segments I–V damaged, and it was not possible to observe setae. Dorsal parts of abdominal segments VI–VIII each with one seta located posteriorly (d1) and five pairs (d2–6) located along their anterior margins; setae short, hair-like. Abdominal segments V–VIII with groups of two lateral setae and one (or two) ventral setae. Abdominal segment IX with two ventral microsetae and two short, thin setae. Pseudocerci absent.

Habitats. Metadonus vuillefroyanus occurs only at saline sites in the presence of its host plant. This species has been observed both at saline sites near the sea (locality Cabo de Gata less than 1 km from the seashore, Fig. 14) and at inland saline sites (Tabernas is approximately 50 km inland from the sea, Fig. 15). The population density of the host plant was not high at these localities; at Cabo de Gata it was much less numerous than the dominant Salicornia maritima, and in Tabernas only several plants were observed. The ruderal sites containing the host plant have also been controlled but without any observations of M. vuillefroyanus.

Adult behaviour. Adults are only occasionally present on host plants during the day; most of them are hidden under the plant and are active at night.

Host plant. Both adults and larvae were observed feeding exclusively on Suaeda vera (Fig. 16). The larvae were found directly on the plants during the day, usually near the growth terminals. Adults and larvae feed on the leaves of the host plant. Larvae, with their cryptic pattern, resemble the colouration of the leaves (Fig. 16). The larvae were found on larger (more than 30 cm high) bunches of the host plant and they usually settled on those with thick, old, dry branches. The adults probably do not migrate long distances and remain under the plant or group of plants at their site.

Life cycle. At the beginning of April, 15 mature larvae and five younger larval instars were swept from host plants. At that time, the host plants had quite young fresh leaves, so the development of these individuals probably began a few days or weeks earlier in the spring so larvae would emerge on the young leaves of the host plant. We suppose the whole larval development to be short-lasting, approximately three weeks at most. The larvae were kept in small plastic containers (height 12 cm, width 4 cm) with small holes for air circulation and exchange. Four to five larvae were kept in each container and no aggressive behaviour was observed, as is known in some Hyperini, e.g., Brachypera vidua (Gené, 1837) (Skuhrovec et al. 2015b). The larvae started pupation early, usually within 2–3 days. The bottom of each container included a 5 cm high level of substrate (collected directly at the locality under the host plants). The majority of the larvae spun their cocoons on the bottom of the container; therefore, they probably generally pupate less than 5 cm into the ground in natural conditions. However, several larvae made the cocoons just below the substrate level, so there seem to exist differences in the pupation preferences (Fig. 18). These cocoons were usually under dead chunks of Suaeda provided for feeding. None of the larvae pupated on or above the ground. The process of creating the cocoon took approximately 1–3 days. Subsequently, larva stayed in the cocoon until pupation, which occurred after 2–4 more days. Most of the larvae moulted into pupae within 5–7 days after entering the substrate. Adults began to emerge 12–18 days later. Altogether, seven adults emerged (five males and two females) by biting a hole in the cocoon. The new adults stayed on the ground surface or remained between a few millimetres and 3 cm above the ground. They were light and smooth at first, but soon acquired the general appearance of normal mature insects, even though it took 3–5 days for them to fully sclerotize. The adults fed on thawed host plant material brought from Spain, but the plants were damaged and did not supply sufficient food. Due to the absence of these host plants in the Czech Republic, we did not try to breed the insects and obtain eggs for a new generation.

The pupae have a green colouration that changes to a brownish colour approximately five days after the start of pupation (Fig. 19). The colouration changes over the entire body; only the eyes are slightly darker. The adults are dark brown (in contrast to the sandy light brown of newly emerged imagines) by 10 days after pupation and are ready to emerge. The emerging adults usually stay in the cocoon for 1–2 days until they are partly sclerotized and then come out and feed on the host plant. The cocoon has two silk layers. The inner layer protects the pupa and is made only of silk. The outer layer includes ground particles.

Biotic interactions. Several of the larvae did not pupate, but larvae of parasitoids emerged and formed typical dark-brown puparia with a whitish band (Figs 20–21).