Ilyobius chilensis (McLachlan)

Berrondo, Matias, 2017, Description of the larva of Ilyobius chilensis (McLachlan) (Megaloptera: Sialidae) and notes on the adult morphology, Zootaxa 4318 (1), pp. 177-186 : 183-185

Notes on adults of Ilyobius chilensis (McLachlan)

Adult illustrations in Flint (1973) are more schematic and those of Contreras-Ramos (2008) are based on dry specimens, with genitalia and eversible sac retracted. Here, we include images of the genitalia with the eversible sac protruded ( Fig. 25 ) and retracted ( Fig. 26 ). We also include a ventral view of male and female apex of abdomen ( Figs. 27 and 28 ). The coloration pattern of head follows that described by Flint (1973) and Contreras-Ramos (2008), but in a few specimens it is less extensive. The description of Contreras-Ramos (2008) is adequate but in fresh specimens an ovoid, membranous, well developed lobe is observed between ectoprocts ( Figs. 25 and 26 ).

Bionomical notes

The species is widespread and abundant in western Chubut Province. Larvae and adults are common in small rivers and montane creeks with muddy bottoms (e.g. “canales de Doña Rosa” and Nant y Fall Creek), or in the slower section of larger rivers (e.g. Río Grande, Río Rivadavia).

Larvae were collected from two microhabitats, at the margins of streams burrowing among instream vegetation or occurring in leaf packs at the bottom of small creeks. Larvae of different instars were collected throughout the year, suggesting a semivoltine life cycle. Larvae prey on small dipteran larvae and culicomorph head capsules (Fig. 15) have been recovered from the gut contents (the digestive tract has a well-developed proventriculus bearing several sharp teeth, (Fig. 16)). Larvae probably prey on other small aquatic arthropods. This information is in agreement with other studies of sialid larvae. Sialis cornuta Ross, 1937 has a two-year life cycle (Pritchard & Leischner, 1973) and their larvae feed mainly on chironomids, tubicid worms, and ostracods. Two other western Nearctic species, S. californica Banks, 1920 and S. rotunda Banks, 1920 either are univoltine or semivoltine ( Azam & Anderson 1969). The life cycle of a Palearctic species, S. lutaria (Linnaeus, 1758) has been reported to have a three-year life cycle, and this species also feeds on small crustaceans, chironomids, and oligochaetes. We were unable to establish the total number of larval instars because of a lack of a suitable sample size and no first instars were collected.

Adults were observed flying between 9:00 am and 3:00 pm. Several adults were observed resting on apices of reeds ( Cyperaceae ), facing down with head and thorax touching the stem and the abdomen at an angle. Adults were active from November to January, with one adult recorded in March (Poncho Moro Stream).

Discussion

Alderfly larvae are poorly known. Of the eight extant genera of alderflies, only two are known from the Neotropical region: Ilyobius , widespread in the Neotropics, and Protosialis , with a single species in Cuba and two others distributed in the Nearctic Region. Larvae of Protosialis have not been described but Bowles & Sites (2015) illustrate the mandible; those of Ilyobius are only known from habitus illustrations or images ( Flint 1973; Azevêdo 2009; Clavier et al. 2010). Other genera for which larvae have been described are Indosialis Lestage ( Bowles & Contreras-Ramos 2016) from the Oriental region; Leptosialis Esben-Petersen ( Crass 1949; Price et al. 2012) from South Africa and Sialis (Leischner & Pritchard 1973; Elliot et al. 1979; Jung & Bae 2012; Jung et al. 2016). Considering that only half of the extant genera larvae are known and that all known sialid larvae are similar morphologically, it is problematic to find diagnostic larval characters for generic characterizations. Nevertheless, we present a comparative listing of characters that we consider useful to differentiate known larvae at the generic level ( Table 1). Larvae of Protosialis , not included in the table, can be easily separated from all other known sialid larvae because their mandibles have three distinctive preapical teeth ( Bowles & Sites 2015), larvae of the other known sialid genera bear two preapical teeth.

We can provide a more detailed comparison between larvae of Ilyobius and Sialis since we had access to larval material of an unidentified Nearctic Sialis species. We found several morphological and morphometrical differences in the head capsule and the head appendages. The most evident difference is the length of the median projection of the labrum, long in Ilyobius ( Fig. 6 , ratio width/length= 1.22) and short in Sialis ( Fig. 17 , ratio width/ length= 1.52). The length of the apical stout setae on the labrum are also different, Sialis has both setae of the same length ( Fig. 18 ) while Ilyobius has one pair short and the other longer ( Fig. 7 ). In the antennae the first antennomere is shorter in Sialis ( Fig. 19 , as long as wide) while it is longer than wide in Ilyobius ( Fig. 8 ); the second antennomere is distinctly longer than the third and fourth in Ilyobius while it is not distinctly longer in Sialis . In the maxilla of Ilyobius the second palpomere is slightly longer than the third and fourth palpomeres combined ( Figs. 11 , 13), while in Sialis the second palpomere is distinctly longer ( Figs. 22, 24 ). The structure of the labium is also useful, in Sialis the prementum is much wider than the length of the prementum and ligula combined ( Fig. 23 ); in Ilyobius the prementum is slightly wider than the length of the prementum and ligula combined (Fig. 14). The mandibles are similar, the only difference we could find is in the size of both inner retinacula; larger in Sialis ( Figs. 20, 21 ) than in Ilyobius ( Figs. 9, 10 ). Finally, the maculation on the head capsule and the pronotum is strongly defined in Ilyobius , in our specimens of Sialis these maculations are less defined, but they are more strongly defined in Palaearctic species ( Elliott et al. 1979).

This paper adds to the knowledge of sialid larvae. However, larvae of four of the genera are still unknown. Despite that all known sialid larvae are relatively similar morphologically; we did find several morphometric characters useful to distinguish known genera. More detailed descriptions of other sialid larvae will surely provide additional characters to diagnose larvae at generic and specific levels.

Figures

FIGURES 25 – 28. adult of Ilyobius chilensis. 25) Male genitalia, lateral view; 26) male genitalia, lateral view; 27) male genitalia, ventral view; 28) female genitalia, ventral view. e: ectoprocts, es: eversible sac, gst 9: gonostylus 9, s 9: sternum 9.

FIGURES 6 – 11. mature larva of Ilyobius chilensis. 6) Labrum, dorsal view; 7) detail of apical portion of labrum, dorsal view; 8) antenna, dorsal view; 9) mandible, dorsal view; 10) mandible, ventral view; 11) maxillolabial complex, ventral view. Scale bars, Figs. 6, 8 – 11 = 0.5 mm.

FIGURES 17 – 24. larva of Sialis sp. 17) Labrum, dorsal view; 18) detail of apical portion of labrum, dorsal view; 19) antenna, dorsal view; 20) mandible, dorsal view; 21) mandible, ventral view; 22) maxillolabial complex, ventral view; 23) detail of labium, ventral view; 24) detail of maxilla, ventral view. Scale bars, Figs. 17, 19 – 24 = 0.25 mm.