Description of the immature stages of Syritta flaviventris (Diptera: Syrphidae) and new data about the life history of European species of Syritta on Opuntia maxima

Adults of Syritta flaviventris and S. pipens were reared from larvae collected on decaying platyclades of Opuntia maxima Miller (Cactaceae) from the Spanish Mediterranean coast. The larva and puparium of S. flaviventris, as well as preliminary data about its life cycle are described. The feeding behaviour of the larva in relation to the cephalopharyngeal skeleton morphology is analysed. Based on the present data, a comparative table containing the main morphological characteristics of the immature stages of European species of the genus Syritta is presented.


INTRODUCTION
The genus Syritta comprises 23 species, of which 1 is present only in the Palaearctic region, 12 in the Afrotropical region, 5 in the Australasian and Oceanian regions, and other 5 species are widespread (Thompson et al., 1976;Thompson & Vockeroth, 1989;Peck, 1988;Dirickx, 1994Dirickx, , 1998)).In Europe only S. flaviventris Macquart, 1842 andS. pipiens (L., 1758) have been found, the latter being one of the most common and ubiquitous hoverflies whose larvae do not feed on aphids (Gilbert, 1993).The geographical distribution of S. pipiens is Pa laearctic (widespread in Europe), Nearctic and Oriental.On the other hand S. flaviventris is known from the Medi terranean Basin and the Oriental, Neotropical and Afrotropical regions.In Europe the circum-mediterranean area is the only territory where these species are sympatric.
Although species of Syritta are common and wide spread, they are biologically poorly known.Within the genus Syritta, only the immature stages and life history of S. pipiens have been described (Kruger, 1926;Hodson, 1931;Heiss, 1938;Dixon, 1960;Hartley, 1961).The rearing records indicate that the larva of S. pipiens is saprophagous, occurring in various kinds of decaying mat ter including garden compost heaps, silage and manure but not in ponds or pools (Lunbeck, 1916;Gilbert, 1993;Rotheray, 1993).Information about the habits, biology and ecology of S. flaviventris is virtually lacking.
The objectives of the present study are: (1) to describe the larva (L3) and puparium of S. flaviventris, (2) to dis tinguish between the immature stages of the two Euro pean Syritta species, (3) to relate feeding behaviour to the morphology of the cephalopharyngeal skeleton, and (4) to present rearing data for S. pipiens and S. flaviventris.
Rearing took place in a growth chamber at 16-22°C, 80 ± 5% r. h. with a constant photo-regime of 15L : 9D photoperiod.Plastic cages (43 cm wide, 24 cm deep, 26 cm high) containing decaying platyclades of O. maxima were checked daily and the L3 instars were kept in a cylindrical plastic cage (40 mm high, 80 mm wide) together with small pieces of dry platyclades to fa cilitate pupation.Puparia were placed individually in 55 mm di ameter dishes and inspected daily until the emergence of the adults.
Larvae selected for preservation were L3 instars.Typically, the larvae of this instar have two discs of differentiated cuticle on the dorsal surface of the first abdominal segment.For perma nent preservation, larvae were killed by immersion in cold water and boiled slowly for about four minutes to extend them.After wards, they were preserved in 70% alcohol.
The external morphology of the third instar larva of S. pipiens and S. flaviventris was compared segment by segment.Descrip tions are based on preserved specimens with larval characters checked against living specimens in order to minimise errors due to preservation.Illustrations and measurements (mean ± standard error) were made on preserved material using a binocu lar microscope with an eyepiece micrometer and FSA 25 PE drawing tube.The photographs were taken with a scanning elec tron microscope (SEM) operated at 20 kV.
Terminology used for descriptions of the larvae follows Har tley (1961) and Rotheray (1993), including the term "mandibu lar lobes", although the structures are probably of maxillary origin.The positions of the sensilla were numbered sequentially from the dorsal to the ventral surface for each segment (Rotheray, 1991).
The cephalopharyngeal skeleton was removed from the lead ing ventral edge of the interior of the puparia, and was then placed in warm potassium hydroxide (K.OH) for 3-4 min.Fi nally it was washed in distilled water and preserved in glycerine until examination.Morphological terminology of this stage fol lows Whittington (1994) and Hartley (1963).

Description of immatures stages of Syritta flaviventris Third larval instar (L3)
Length 8-10 mm, maximum width 1.5-2 mm (n = 4).Overall appearance: A short-tailed larva with internal mouth-hooks, bearing three conspicuous pairs of fleshy lappets located before the posterior end of the larva (Fig. 4).Subcylindrical in cross-section with a flattened ventral surface, truncate anteriorly, and slightly tapering posteri orly.Cuticle translucent when alive, cream to off-white after fixation.ITind gut green throughout the translucent body in actively feeding individuals, otherwise hind gut obscured by white adipose tissue.Dorsal body surface coated in short and fine pubescence backwardly directed, longer and slightly sclerotised on the terminal body seg ments.Setae on ventral suface are shorter except for the anal segment (hind end).
T horax.Lateral lips rounded and well developed (in profile projecting forward from the anterior part of the prothorax) and covered with short and fine setae at base and longer and conspicuous spicules at tip.Anterior fold on dorsal surface of prothorax with longitudinal grooves and a narrow band (< 40% of anterior fold) of densely ag gregated, backwardly directed, sclerotised spicules which become progressively shorter posteriorly.Dorsal surface of prothorax with anterior respiratory process weakly sclerotised, short and narrow.Lateral margin of prothorax (P) with two paired patches of sclerotised spicules ar ranged as follows: A group of 7 just anterior to 4th sensillum of mesothorax (Ms) and another prothoracic group of 12 spicules is located in front of 5th sensillum of meso thorax (Fig. 1).Mesothorax bearing well developed pro legs with about 20 crochets.Ventral surface of meta thorax (Mt) with two little patches bearing very small crochets in the position homologous to the mesothoracic prolegs.
A bdomen.Primordia of pupal spiracles distinct on dor sal surface of first abdominal segment.Ventral prolegs (Pr) small, 6 abdominal pairs on segments 1-6.Each pro leg with 2 or 3 rows of apically brown crochets: 5 or 6 primary, 4 or 5 secondary and some tertiary.Crochets in anterior row slightly bigger than those posterior.Crochets are directed posteriorly except on the last abdominal seg ment where some are anteriorly directed.Anal segment with three pairs of fleshy lappets, last pair bigger than the anterior ones (Fig. 4).
Puparium.Length including posterior respiratory proc ess 6.5 mm, maximum width 2.5 mm (n = 3).Subcylindrical in cross-section.Anterior extreme rounded, tapered posteriorly and flattened slightly ventrally (Fig. 8).In tegument rough, with segmentation of larvae persisting as transverse folds and wrinkles.Ventral surface with pro legs visible on mesothorax and first 6 abdominal seg ments.Dorsal ly, the two thoracic respiratory processes protrude on the upper half of the operculum (Fig. 10).These processes are subconical structures, swollen and lobulatcd apically (Fig. 12).Basal part of the thoracic res piratory processes nodulated on the two basal thirds, with a small smooth area just below the swollen apex.Lobulated area presents numerous disc-shaped spiracular open ings, distributed irregularly on the surface (Fig. 14).Each disc has from 2 to 6 oval apertures.

Life history
Adults of Syritta species were repeatedly observed fly ing around the platyclades of O. maxima.Examination of these platyclades revealed that the larvae and puparia were confined to decaying platyclades.
The life cycle is completed inside decaying platyclades that characteristically occur on the branches of O. maxima, and also on detached and fallen platyclades.Pu paria were always found inside old dried platyclades where the decaying process was complete.
Adults of S. pipiens could be found throughout the year.Larvae and puparia of this species were found from early winter to summer (December to August) (Fig. 16).The puparium period was 8-11 days (n = 20).
The imagines of S. flaviventris were scarce and ob served only from February to March.Larvae and puparia of this species were found during early winter (January and February) (Fig. 16).Pupal period was 8-10 days (n = 3).

DISCUSSION
The co-occurrence of larvae of Syritta with larvae of other hoverflies has also been observed in various other Figs 12-15.12, 13 -thoracic respiratory process of puparium.12-5.flaviventris; 13-5.pipiens.14, 15 -spiracular openings.14 -5.flaviventris; 15-5.pipiens.decaying materials.Hodson (1931) (1994) found one larva of S. pipens on an artichoke root attacked by Cheilosia vulpina, probably using the rotting plant debris as food source.Rotheray (1993) provides a key to European hoverfly genera based on third-instar larvae.In this key the identi fication characters for the genus Syritta are based exclu sively on larvae of S. pipens.These morphological characters (prolegs small with 5 or 6 primary crochets ar ranged as a transverse row across the segment; anal seg ment with 3 pairs of lappets, the first two pairs equal in length and third pair longest at tip) are shared by the lar vae of S. flaviventris.
As could be expected, the larvae of S. flaviventris and S. pipens show striking similarities.However, close ex amination revealed slight but obvious differences be tween both species.Therefore, the larvae of the genus Syritta may be distinguished according to the characteris tics summarized in Table 1: Ornamentation of dorsal body surface and shape of the slits on posterior respira tory process.
The main diagnostic characters of the puparium are: (1) shape of thoracic respiratory processes, (2) ornamentation of thoracic respiratory processes, and (3) shape of spiracular openings of thoracic respiratory processes (Table 1).
The morphology of the cephalopharyngeal skeleton of S. flaviventris is very similar to S. pipens (Figs 2, 3), both species have well developed ventral pharyngeal ridges.The ventral pharyngeal ridges select the appropriate food particle size, permitting partial digestion before swallow ing of the particles and increasing efficiency of food utili sation by its concentration (Roberts, 1969).Larvae pos sessing pharyngeal ridges are essentially particle feeders, reducing the amount of non-nutritious liquid ingested and concentrating food (consisting of a suspension of bacteria, protozoa, fungal spores and yeast) by sieving (Dowding, 1967).In conclusion, the morphological study together with the rearing data of S. flaviventris reveal saprophagous feeding habits within decaying platyclades of Opuntia cacti.Hartley (1963) reported that differences in diet between taxa are mainly reflected by differences in relative pro portions and sclerotisation of the sclerites of the cephalo pharyngeal skeleton.Since clear morphological dif ferences between the cephalopharyngeal skeleton of S. flaviventris and S. pipiens are lacking, this may indicate that there are no differences in diet between these two taxa.The fact that the larvae of both species were found breeding together supports this hypothesis.Moreover, prickly pear cacti were not introduced into the distribu tion area of S. flaviventris until the 16th century, and the relationship between this host plant and S. flaviventris lar vae should be a secondary adaptation.Therefore, the lar vae of S. flaviventris may be found in other kinds of de caying matter similar to the breeding media employed by 5. pipiens.
ACKNOWLEDGEMENTS.We thank G. Rotheray for critically reviewing an earlier draft of this manuscript.The illustrations were kindly produced by M. Luz Roca.Financial support was provided by the Generalität Valenciana (GV-C-RN-12-069/96) and the Ministerio de Educación y Ciencia (PB 96/01413).Covered with short and fine pubescence backwardly directed, longer and slightly sclerotised on the last segments Clearly sinusoidal (Fig. 6) Swollen apically with lobulated profile (Fig. 12) Nodulated but with a smooth area just below the swollen area (Fig. 12) Spiracular openings on a disc-shaped base (Fig. 14) Covered with short and fine pubescence backwardly directed Only slightly sinusoidal (Fig. 7) Swollen apically with smooth profile (Fig. 13) Nodulated from base to just below the swollen area (Fig. 13) Spiracular openings directly on the surface of swollen area (Fig. 15)

Fig
Fig. I. Map of the chaetotaxy of the third instar larva of S. flaviventris showing the positions of the sensilla group on: P -pro thorax; Ms -mesothorax; Mt -metathorax; Al, A7 -first and seventh abdominal segments; A8 -anal segment (hind end); Nsnonfunctional spiracle; LI -lateral lip; Pr -proleg; Sp -anterior functional spiracle; W -area of differentiated cuticle through which the pupal spiracle will be thrust.
Fig. 4. S. Jlaviventris, anal segment of puparium in dorsal view showing three pairs of lappets.

Table 1 .
Differences between the third stage larva and puparium of S. flaviventris and S. pipiens.