Bioacoustics of Holocnemus pluchei

Redescription Male ( Israel , ZFMK Ar 22306) MEASUREMENTS. Total length 6.0, carapace width 2.3. Distance PME–PME 220 µm; diameter PME 140 × 150 µm; distance PME–ALE 70 µm; diameter AME 125 µm; distance AME–AME 25 µm. Leg 1: 45.4 (12.9 + 1.1 + 12.7 + 16.4 + 2.3), tibia 2: 8.1, tibia 3: 6.0, tibia 4: 7.2; tibia 1 L/d: 42; femora 1–4 diameters: 0.46, 0.39, 0.37, 0.38. COLOR (in ethanol). Carapace ochre-yellow, posterior part of ocular area brown; clypeus not darkened; sternum dark brown to black; legs ochre-yellow, with darker rings on femora (subdistally) and tibiae (proximally and subdistally), with oval to elongate black marks on femora and tibiae; abdomen ochregray, with dark and whitish marks dorsally and laterally; ventrally with distinct black median band, partly disrupted, with three parallel longitudinal marks behind gonopore. BODY. Habitus as in Fig. 5 . Ocular area slightly raised; each secondary eye with small accompanying elevation ( Fig. 43 ). Deep thoracic pit and pair of shallow furrows diverging from pit toward posterior margin. Clypeus unmodified (contra Calbacho-Rosa et al. 2019b ; see Discussion), only rim more sclerotized than in female. Sternum wider than long (1.5/1.1), unmodified (i.e., without indentations as in H. caudatus and H. reini ). Abdomen cylindrical, dorso-posteriorly weakly angular. Gonopore with four epiandrous spigots ( Fig. 55 ). ALS with one widened spigot and one pointed spigot; PMS with two pointed spigots ( Fig. 56 ). CHELICERAE. As in Fig. 16 ; see also Huber (1995 : figs 1a, 4a; 2000: fig. 14); with pair of frontal lateral apophyses, each with one large modified cone-shaped hair ( Fig. 54 ); distance between tips of modified hairs: 390 µm; without proximal frontal protrusion; lateral stridulatory ridges distinct ( Fig. 47 ), distances between ridges ~12 µm. PALPS. As is Figs 13–15 ; coxa with rounded retrolateral hump (not a distinct apophysis); trochanter barely modified (slightly protruding ventrally); femur curved towards dorsal, distally widened but without ventral protrusion, proximally with prolateral stridulatory pick, without retrolateral transversal line, with prominent retrolateral proximal process; femur-patella joints only slightly shifted toward prolateral side; tibia large compared to femur, tibia-tarsus joints shifted toward retrolateral side; tarsal organ capsulate ( Fig. 51 ); tarsus without macrotrichia; procursus ( Figs 20–22 ) straight, dorsal hairs not or only slightly curved upwards; proximally on prolateral-ventral side with prominent process free of hairs (arrow in Fig. 20 ), procursus tip with strong but short ventral sclerite, membranous elements on dorsal and prolateral side, and 4–5 hair-like transparent processes on retrolateral side ( Fig. 49 ); genital bulb ( Figs 23–26 ) with simple basal sclerite connected to distal (main) sclerite; distal sclerite with two distinctive processes; sperm duct opening on membranous area on prolateral side at basis of distal bulbal sclerite (arrow in Fig. 50 ). LEGS. With single ventral rows of spines on femur 1 (~31), tibia 1 (~38) and femur 2 (~14) ( Figs 17– 19 ); without curved hairs; few vertical hairs; retrolateral trichobothrium of tibia 1 at 4%; prolateral trichobothrium present on all tibiae; tarsal pseudosegments indistinct and irregular (cf. Fig. 35 ) except 2–3 distally; all tarsal organs capsulate with simple round rim ( Fig. 53 ). Male (variation) Tibia 1 in 95 males : 7.9–15.5 (mean 11.6) (mean tibia 1 length in 204 males in Jakob & Dingle 1990 : 10.67); body length ~3.5–7.5. Most specimens with more or less distinct dark median band on carapace; sternum sometimes with distinct black radial marks; abdomen sometimes without dark marks, but always with white marks in distinctive pattern. Smaller males with fewer spines, often without spines on tibia 1 and femur 2. Distance between tips of modified hairs on cheliceral apophyses: ~280–390 µm. Gonopore with 4–5 epiandrous spigots. Female In general similar to male ( Fig. 6 ) but palp with strongly widened distal segments (tibia and tarsus; Figs 29–30 ), without joints between tibia and tarsus; with strong median process posteriorly on sternum ( Figs 31 , 58 ) possibly acting against ventral process anteriorly on abdomen ( Figs 38 , 57 ; but see Discussion); with fewer but stronger stridulatory ridges ( Fig. 48 ; see also Huber 1995 ; Huber 2021a : fig. 12), distances between stridulatory ridges ~13.5 µm; without spines on legs; dark marks on femora and tibiae ( Fig. 34 ) often more distinct than in males; with strong anterior sclerite dorsally on pedicel possibly acting against pair of sclerotized areas anteriorly on abdomen ( Fig. 32 ). Without stridulatory apparatus between carapace and abdomen. Tibia 1 in 140 females : 7.2–14.1 (mean 10.5) (mean tibia 1 length in 172 females in Jakob & Dingle 1990 : 10.05); body length: ~4.0–8.0 mm. Epigynum as in Figs 36–39 and 57 ; main epigynal plate wide, weakly protruding, posteriorly whitish, anteriorly with pair of depressions ~280–300 µm apart, with sculptured cuticle (cf. Huber 1995 : fig. 6c), median internal round element and pair of lateral internal triangular elements usually visible in uncleared specimens; posterior epigynal plate large but simple; large anterior plate in front of epigynum with pair of low humps and distinct anterior elevated rim (arrow in Fig. 38 ). Internal genitalia ( Figs 27–28 , 40–42 ) with elongate pore plates in transversal position narrowing medially, dorsal arc relatively simple, ventral arc with distinctive pair of lateral sclerotized pockets, medially widened, with ventral median process (pocket?). Spigots as in male. Natural history Together with Pholcus phalangioides (Fuesslin, 1775) , H. pluchei is probably the best studied pholcid spider with respect to its biology. Much of what we know about H. pluchei has been studied in the USA and Argentina , i.e., using non-native populations. Whether they differ in any significant way from Mediterranean populations has never been studied. Data on egg numbers (see below) suggest that such differences may exist. Around the Mediterranean, H. pluchei is ubiquitous in almost any habitat suitable for its web, including places fully exposed to the sun. Occasionally, H. pluchei lives in peripheral parts of webs of the araneid Cyrtophora citricola ( Forsskål, 1775 ), together with the cobweb spider Argyrodes argyrodes (Walckenaer, 1841) ( Blanke 1972 ; Hajer 1995 ; Leborgne et al . 1998 ; Hajer & Řeháková 2003 ). In Central Europe, where H. pluchei has been massively spreading over the last decades, it may be displacing P. phalangioides , at least in warm/dry/light parts of buildings, close to windows ( Jäger 2000 ; Van Keer 2007 ). In the southwestern USA , H. pluchei is often the dominant spider on outside surfaces in urban areas around homes and other structures ( Vetter et al. 2011 ). It sometimes occurs is high densities: up to 600 spiders were estimated to inhabit a 3 × 15 m juniper bush in California ( Blanchong et al. 1995 ). The usual web of H. pluchei is dome-shaped, as in most space-dwelling pholcids. A skeleton made of 1.2 µm fibrils is filled with 0.8–0.9 µm fibrils ( Hajer & Řeháková 2003 ). Occasionally, webs are densely covered with silk puffs, a common behavior in Smeringopinae that interferes with a clear view of the spider. The fact that silk puffs are produced 3–4 days before molting and before egg-laying supports the view that puffs may protect the spider from visual predators when it is most vulnerable ( Hajer & Řeháková 2003 ). Unusual spherical webs are built by females before egg-laying. Females remain in these small spheres (diameter ~ 5 cm ) until the spiderlings hatch, and females do not prey during this time ( Sedey & Jakob 1998 ; Jäger 2000 ; Hajer & Řeháková 2003 ). Holocnemus pluchei is facultatively group-living , i.e., several conspecifics of all sizes may share a web ( Jakob 1991 ). Groups are constantly forming and disintegrating as the spiders move frequently among webs, depending on size, feeding status, and presence or absence of conspecifics ( Jakob 2004 ). Surprisingly, small spiders in a group seem to pay a high prize for group living: they feed less than solitary siblings and larger companions, they tend to lose contests, and they are sometimes cannibalized ( Gerhardt 1927 ; Jakob 1991 ; Jakob et al. 2000 ). The reason they join conspecifics may be the cost of building an own web: it takes a spiderling at least 5 days to feed enough to build a web ( Jakob 1991 ), and webs are not recycled ( Jakob et al. 2000 ). Sperm uptake was described by Gerhardt (1927) . First, a line of silk between the tips of legs 3 is moved against the gonopore until the sperm droplet emerges. Then the line with the drop is brought to the chelicerae from where it is taken up by the genital bulbs. In alternating movements, each bulb contacts the drop three times for approximately 30 s each. Within the bulb, sperm remains viable at least for several weeks (99.6% viable after two weeks; Cargnelutti et al. 2020 ). Basic aspects of courtship and copulation have been described by Gerhardt (1927) , Huber (1995) , and Dutto et al. (2011) . Both males and females (and juveniles) use their chelicerae to stridulate, but in different contexts. While male stridulation seems to have a luring function, female stridulation conveys a negative message, mainly to males (signaling non-receptivity), but also to other females and maybe even to other species (e.g., the web-invading Pholcus phalangioides ) ( Huber 1995 ; Dutto et al. 2011 ). The stridulation of juveniles has never been studied. Female stridulatory behavior has been found to be consistent, i.e., individual females differ consistently in the frequency of stridulating during consecutive inter-sexual encounters ( Calbacho-Rosa et al. 2019a ). In copulations with virgin females, sperm is transferred during a first phase that is characterized by rhythmic simultaneous movements of the male palps ( Cargnelutti et al. 2018 ). This is followed by a second phase where the palps remain inserted but seemingly immobile. The significance of this second phase is only partly understood. It might be a form of mate-guarding, and the duration of the immobile phase positively affected sperm viability in females (which decreased more rapidly in females than in males) ( Cargnelutti et al. 2020 ). Copulations with non-virgin females are common; they take longer (~40 vs 30 min; Kaster & Jakob 1997 ), and they follow a slightly different pattern. They start with non-rhythmic alternating movements of only partially inserted palps (only procursus inserted) during which sperm of previous males is partially removed ( Calbacho-Rosa et al. 2013 ). Second males fertilize an average of ~65–83% of the eggs, but the actual values range from 0 to 100% ( Kaster & Jakob 1997 ). Second-male sperm precedence depends on timing: it is strongest in the few hours after copulation ( Calbacho-Rosa et al. 2010 ) (explaining the pattern of mate guarding; see below). The projection on the female sternum has been interpreted as a structure that controls the intensity and range of male palpal movements during copulation ( Calbacho-Rosa et al. 2019b ). These authors support their conclusion mainly by the fact that the projection contacts the male clypeus at the moment of maximum palpal contraction. They exclude a stidulatory function (with the “pre-epigynum”) because the structures do not contact each other during intersexual interactions (but see Discussion). The biological significance of the dorsal female modification (pedicel, abdomen) has never been studied (it has apparently not even been described before). It may function during abdominal twitching, a component of low-level aggressive interactions ( Jakob 1991 , 1994 ). Abdomen twitching was the only antagonistic behavioral component that differed between sexes, but it was males (that lack pedicelabdomen modifications) that performed more twitches ( Blanchong et al. 1995 ). Escalating fights involve the legs, and Johnson & Jakob (1999) found that 7–8% of spiders in natural populations were missing at least one leg, usually (in 86% of the cases) one of the anterior two pairs. However, leg loss does not significantly affect the male’s ability to compete over webs and prey ( Johnson & Jakob 1999 ). Reports on mate guarding are somewhat contradictory. For Californian populations, Kaster & Jakob (1997) report the absence of guarding; Sedey & Jakob (1998) found that most females with eggs were initially accompanied by at least one male . In Argentinean populations, Calbacho-Rosa et al. (2010) observed post-copulatory mate guarding: males stayed close to females for approximately 12–24 hrs after copulation, and actively defended females (or rather their own investment) from intruding males. This does not seem to imply chivalrous behavior: males do not cede prey to females, and they win interactions as often as females do ( Blanchong et al. 1995 ). Females produce several egg-sacs per season, and Hajer & Řeháková (2003) reported a maximum of eight clutches in the lab. As usual in pholcids ( Huber & Eberle 2021 ), larger females lay more eggs (and thus produce heavier clutches), but egg weight is not affected by female body size ( Skow & Jakob 2003 ). Kaster & Jakob (1997) , working on Californian populations, reported a mean clutch size of 33 (9–70), while Ahmed (2021) , working on Egyptian spiders, reported a mean of 89. While Ahmed’s (2021) publication is flawed in many respects, this number is not unfeasible. A single egg-sac of a female from Croatia reported in Huber & Eberle (2021) was estimated at having 77 eggs . While caring for an egg-sac, females are reluctant to remate, but occasionally they do, especially at a late stage of embryo development ( Calbacho-Rosa et al. 2017 ). In order to mate, they have to temporarily suspend the egg-sac in the web, which supposedly carries some risk for the eggs, especially from (conspecific) predators ( Calbacho-Rosa et al. 2017 ). Egg-sac carrying has also been shown to protect the eggs from fungi ( Calbacho-Rosa et al. 2017 ), but this is not likely to have a significant effect during the relatively short time of mating. Developmental time depends on food level, and well-fed spiders often reach maturity after five molts, while poorly-fed spiders usually need six molts ( Jakob & Dingle 1990 ). However, spiders maturing after six molts are on average larger, irrespective of food level ( Jakob & Dingle 1990 ). When legs were removed at the third instar, they were not regenerated ( Johnson & Jakob 1999 ). When disturbed , H. pluchei starts to move vigorously like many long-legged pholcids. However, the movement has been described as ‘bouncing’, different from the ‘whirling’ of P. phalangioides ( Jackson et al. 1993 ) . When bouncing, H. pluchei lifts and lowers the body by flexing and extending the femurpatella and tibia-metatarsus joints at an amplitude of 2–20 cm and at a rate of 5–10/s ( Jackson et al. 1993 ). This is thought to protect the spider from visual predators, and it may also interfere with a predator’s movement in the web ( Jackson 1992 ; Jackson et al. 1993 ). An alternative strategy is to leap out of the web and ‘play dead’ ( Jackson et al. 1993 ).