Immature stages , morphology and feeding behaviour of the saprophytic syrphids Copestylum tamaulipanum and Copestylum lentum ( Diptera : Syrphidae )

Adults of Copestylum tamaulipanum and C. lentum were reared from larvae collected from decaying platyclades of the cactus Opuntia in the state of Veracruz (Mexico). The larvae and puparia of both species, as well as preliminary data about their life cycles are described. The feeding behaviour of the larva in relation to the morphology of the cephalopharyngeal skeleton is analysed.

Of these 12, only four: C. apicale (Loew, 1866), C. vitatum (Thompson, 1887), C. mexicanum (Macquart, 1842) and C. vacuum (Fabricius, 1775) have been found and reared in rot-pockets of Opuntia species.The two species studied in this paper were found breeding in decaying platyclades of Opuntia species in Veracruz, Mexico, and represent the first biological data on these species.
The objectives of the present study are: 1) to describe the larva (L3) and puparium of Copestylum tamauli panum (Townsend, 1898) and Copestylum lentum Willis ton, 1887, 2) to compare the immature stages and mor phology of the cephalopharyngeal skeleton of all Cope stylum species reared on Cactaceae and 3) to present developmental data for these two species.

MATERIAL AND METHODS
Two adults of C. lentum and seven of C. tamaulipanum were reared from larvae collected on different species of Opuntia growing at different localities in the state of Veracruz (Gulf of Mexico).
Larvae were reared in a growth chamber at 16-22°C, 80-85% r h with a constant photo-regime of 15L : 9D photoperiod.Plastic cages (30 cm wide, 15 cm deep, 20 cm high) containing decaying platyclades of Opuntia were checked daily and L3 instars were placed in a cylindrical plastic cage (40 mm high, 80 mm wide), together with small pieces of dry platyclades, to facilitate pupation.Puparia were placed individually in 55 mm diameter dishes and inspected daily until the emergence of the adults.
Third instar larvae were selected for preservation.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.Descriptions are based on preserved specimens, and the larval characters checked against living specimens in order to mini mise errors due to preservation.Illustrations and dimensions (mean ± standard error) were measured on preserved material using a binocular microscope with an eyepiece micrometer and FSA 25 PE drawing tube.The photographs were taken with a scanning electron microscope (SEM) operated at 20 kV.
Terminology used in the descriptions of the larvae follows Hartley (1961) and Rotheray (1993).The positions of the sen silla were numbered sequentially from the dorsal to the ventral surface on each segment (Rotheray, 1991).
The cephalopharyngeal skeletons were removed from the leading ventral edges, of the interior of the puparia, and placed in warm potassium hydroxide (KOH) for 3-4 minutes.Finally they were washed in distilled water and preserved in glycerine until examined.Morphological terminology of this structure fol lows Hartley (1963).
Voucher specimens of the adult and immature stages of C. tamaulipanum and C. lentum are in the Entomological Collec tion of Alicante University, Smithsonian Institution of Wash ington and in the National Museum of Scotland.

Description of the immature stages
Copestylum tamaulipanum (Townsend, 1898) (Figs 2A,(3)(4) Third larval instar (L3) Length 18.2 ± 0.62 mm, maximum width 3.1 ± 0.12 mm (n = 6).Overall appearance: A short-tailed larva with internal mouth-hooks, bearing two pairs of fleshy lappets located anterior to the posterior end of the larva.Subcy lindrical in cross-section with a flattened ventral surface, slightly truncate anteriorly, and tapering posteriorly.Cuticle translucent when alive, cream to off-white after fixation.Dorsal body surface coated in short, pointed and unpigmented backwardly directed spicules, which are shorter and scarce on the ventral surface except for the anal segment (hind end).
Thorax.Lateral lips rounded and well developed (in profile projecting forward from the anterior part of the prothorax), and coated in long and fine unpigmented setae (Fig. 3B).Dorsal lip coated in very short, pointed and sclerotised spicules.Anterior fold on dorsal surface of the prothorax with longitudinal grooves and narrow band (< 30% of anterior fold) of densely aggregated, backwardly directed, sclerotised spicules that become progressively shorter posteriorly.Dorsal surface of the prothorax with anterior respiratory process sclerotised, short and trilobed apically.Lateral margin of prothorax (P) with two patches of sclerotised spicules arranged as follows: a group of 15-20 just anterior to 4th sensilla of mesothorax (Ms) and another prothoracic group of 15-20 spiculae is located in front of the 5th sensillia of meso thorax (Fig. 1).Dorsal surface of mesothorax with a patch of 15 sclerotized spicules behind anterior respiratory process.Mesothorax bearing well developed prolegs with about 50 crochets each arranged in two semicircular groups (Fig. 3A).Ventral surface of metathorax (Mt) with a narrow band bearing about 80-85 small and slightly sclerotized crochets (Fig. 3A).
Abdomen.Primordia of pupal spiracles obvious on the dorsal surface of first abdominal segment.Ventral prolegs (Pr) small and multi-serial, 6 abdominal pairs on seg ments 1-6.Each proleg with 35-38 brown crochets.Cen tral crochets slightly bigger than the anterior and posterior ones, all of which are directed posteriorly (Fig. 3C).Anal segment with two pairs of fleshy lappets, posterior pair located ventrolaterally, just anterior to base of the poste rior respiratory process (prp); second and smaller pair located dorsolaterally and anterior to preceding pair (Fig. 3D).

Puparium
Length including posterior respiratory process 8.2 ± 0.21 mm, maximum width 3.14 ± 0.09 mm (n = 7).Sub cylindrical in cross-section (Fig. 4A).Anterior extremely truncated, tapered posteriorly and flattened ventrally.Integument rough, with segmentation of larvae persisting as transverse folds and wrinkles.Brown in colour.Ven tral surface with prolegs visible on mesothorax and first 6 abdominal segments.Dorsally, two thoracic respiratory processes protrude on the upper half of the operculum, they are sep rated by a distance of about one and half time their length (Fig. 4B).These processes are subcylin drical structures of about 0.8 mm in length, bearing a crown of irregularly-spaced and rounded tubercles usu ally extending no more than a third of the way down the upper surface (Fig. 4C).Surface between tubercles smoothly-polished with very fine short and scattered setae, except around apical tubercules where it is finely beaded (Fig. 4D).Each tubercle has from 2 to 5 oval openings (Fig. 4D).Basal part of the thoracic respiratory processes with a granular surface (Fig. 4E).Base of the processes encircled by pointed spicules (Fig. 4F).
Copestylum lentum Williston, 1887 (Figs 2B,(5)(6) Third larval instar (L3) Length (tip of prothorax to apex of the posterior respi ratory process) 15.3 ± 1.76 mm; maximum width 3.6 ± 1.33 mm (n = 3).Overall appearance: A short-tailed larva with internal mouth-hooks, bearing three pairs of fleshy lappets, second pair reduced to just sensilla and surrounding setae, and first and third of similar size.Sub cylindrical in cross-section with a flattened ventral sur face, truncate anteriorly, and slightly tapering posteriorly.Cuticle translucent when alive, cream to off-white after fixation.Dorsal body surface coated in short, pointed and unpigmented spicules backwardly directed that are shorter on the ventral surface except those on the anal segment.
Thorax.Lateral lips rounded and well developed (in profile projecting forward from the anterior part of the prothorax) and coated in long and fine unpigmented setae.Dorsal lip coated in very short, pointed and sclero tised spicules (Fig. 5A).Anterior fold on dorsal surface of the prothorax with longitudinal grooves and narrow band (< 45% of anterior fold) of densely aggregated, back wardly directed, sclerotised spicules that become progres sively shorter posteriorly.Anterior respiratory processes lacking, their position marked by two minute, circular areas on dorsal surface of the prothorax.Lateral margin of mesothorax (P) with two patches of sclerotised spic ules arranged as follows: a group of 20-25just anterior to 4th sensilla of mesothorax (Ms) and another prothoracic group of 13-18 spiculae located in front of the 5th sensilla of mesothorax.Dorsal surface of mesothorax with a patch of20-25 sclerotised spicules anterior to 1st and 2nd mesothoracic sensilla.Lateral margin of metathorax (Mt) with one group of 10-12 sclerotised spicules surrounding 4th sensillum of metathorax and another group of 6-10 spiculae located in front of the 5th sensillum.Mesothorax bearing well developed prolegs with about 35-40 cro chets, arranged in 3-4 rows (Fig. 5B).Space between prolegs covered in small and sclerotised spicules.Ventral surface of metathorax with a narrow band bearing about 70-75 small and slightly sclerotised crochets.
Abdomen.Primordia of pupal spiracles obvious on dorsal surface of first abdominal segment.Ventral prolegs (Pr) small, 6 abdominal pairs on segments 1-6.Each proleg with 2 or 3 rows of apically brown crochets: 7 or 9 primary, 7 or 9 secondary and some tertiary (Fig. 5C), except on the last abdominal segment where they are absent or reduced in number.Crochets in anterior row slightly bigger than posterior ones.Anal segment with three pairs of fleshy lappets, second pair located ventro laterally and reduced to just sensilla and surrounding setae, first pair located dorsolaterally and the third pair placed ventrolaterally, just anterior to base of the poste rior respiratory process (prp).
Length 1.1 ± 0.09 mm; width: at base 0.61 ± 0.008 mm, at tip 0.47 ± 0.014 mm; (n = 3).Shiny, sclerotised and brown in colour.Spiracular plates fused into a single plate constricted medianly.Six spiracular slits with a clearly sinuous shape arranged around two central scars (Fig. 5E).Periphery with four pairs of long and plumose spiracular setae with at least, five basal branches.Basal two-thirds of the posterior respiratory process finely ridged tojust below the smooth and shiny tip (Fig. 5D).

Puparium
Length including posterior respiratory process 8-9 mm, maximum width 3-4 mm (n = 2).Subcylindrical in cross section.Anterior extremely truncated, tapered posteriorly and flattened ventrally (Fig. 6A).Integument rough, with segmentation of larvae persisting as transverse folds and wrinkles.Brown in colour.Ventral surface with prolegs visible on mesothorax and first 7 abdominal segments.Dorsally, the two thoracic respiratory processes protrude on the upper half of the operculum, they are separated by a distance of about four times their length (Fig. 6B).These processes are sub-cylindrical structures of about 0.4 mm in length, bearing a crown of irregularly-spaced rounded tubercles usually extending no more than a quarter of the way down the upper surface (Fig. 6C).The base is entirely encircled with irregularly spaced and pointed spicules (Fig. 6C).Entire surface, including spaces between tubercles, finely beaded, some of these minute projections bear a single apical seta (Fig. 6D).Each tubercle has from 4to6 oval apertures (Fig. 6D).

Biological data
Larvae of C. lentum and C. tamaulipanum were found inside decaying platyclades that characteristically occur on the branches of Opuntia, and in detached and fallen platyclades.The necrotic areas of these platyclades seem to be produced by microorganismes, maybe a bacterium like that causing the bacterial necroses on the giant saguaro (Cereus giganteus Engl.) (Lightle et al., 1942).The infected region, still covered by the epidermis, becomes water-logged with the destruction of the internal tissues.The necrotic tissue continues to desintegrate, and at the final stage of decay it may become spongy and eventually dry out.The larvae of these two species of Copestylum were found mainly during the later stages of decay when the most of the internal tissues have been reduced to a syrupy malodorous liquid.
Generally, damaged platyclades shelter numerous larvae of the same species of Copestylum together with other larvae of Stratiomyidae.Pupation took place on the dry parts of platyclades where the process of decay was complete.

DISCUSSION
Only the immature stages of a few species of Cope stylum have been described in detail.Based on the descriptions in the literature and our morphological study of the immature stages of C. lentum and C. tamaulipanum, we can conclude that the overall appear ance of the larvae of Copestylum is characterised by: a short-tail, sub-cylindrical in cross-section; mouth-hooks and mandibular lobes internal; prothorax with a band of sclerotised spicules on the anterior fold; prolegs with cro chets arranged in a transverse row; anal segment with lap pets.
As is to be expected, the larvae of Copestylum that breed in decaying Cactaceae show striking similarities.However, close examination revealed differences between the species.Therefore, the larvae of the genus Copestylum that breed in decaying Cactaceae may be dis tinguished by the following characteristics: (1) presence or absence of anterior spiracles; (2) number and relative size of the lappets on the anal segment; (3) shape and ornamentation of the posterior respiratory process and (4) size of the mandibular lobes in relation to the length of the mouthhook (Table 1).
For the puparium, the main diagnostic characters are summarized in Table 2: shape and ornamentation of tho racic respiratory processes and distance between thoracic respiratory processes.
Decaying cactus, whether prickly pear or other species, is a very suitable breeding ground for saprophagous syrphids, mainly Copestylum species but also some species of Nausigaster Williston, 1883 (Hunter et al., 1912;Rotheray et all., 2000.), Eumerus Meigen, 1822and Syritta Lepeletier & Serville, 1828(Pérez-Bañón & Marcos-García, 1998;2000).These syrphids are attracted  (Bugbee & Reigel, 1945;Santana, 1961;Mann, 1969).The morphology of the cephalopharyngeal skeleton of C. lentum and C. tamaulipanum (Fig. 2A and 2B) accords well with their feeding habits, both species have well developed ventral pharyngeal ridges.The ventral pharyn geal ridges select the appropriate food particle size, per mitting partial digestion before swallowing of the parti cles and increasing efficiency of food utilisation by its concentration (Roberts, 1969).The cephalopharyngeal skeleton of the two species described here are very similar and also show striking similarities with the cepha lopharyngeal skeleton of C. vacuum (Fabricius, 1775) (the only Copestylum species whose cephalopharyngeal skeleton has been described).These striking similarities may be explained by the absence of differences in diet between these taxa.
The other mouthparts of the larva of C. lentum and C. tamaulipanum are very similar to those of other saprophagous syrphids such as Eristalis Latreille, 1804, Bra-chyopaMeigen, 1822, Chrysogaster Meigen 1803, Xylota Meigen, 1822 etc. which feed by filtering micro organisms from fluids.For example, in all these taxa: the thorax is broad and the anterior fold is coated in spicules; the dorsal lip is firm not fleshy; the lateral lips are large and coated in variously sized setae; the mandibular lobes are large, consisting of numerous ridges.The functional significance of this morphology appears to be for imbibing and filtering food particles suspended in fluids and expelling the filtered fluid (Rotheray, 1993).
Nevertheless, the food channel, a depressed region pos terior to the mouth, is lacking in the two species here described.This food channel is a particular feature of saprophagous syrphids developing in liquid media.It is also absent in Syritta larvae reared on Opuntia spp., which may be related to the different density of the medium, as the decaying platyclades of Cactaceae offer a thick substratum, which is not completely fluid.a crown of irregularly-spaced and rounded tubercles usually extending no more than a third of the way down the upper surface (Fig. 4C).Surface between tubercles smoothly-polished.Basal part of the thoracic respiratory processes with a granular surface (Fig. 4E).
C. lentum straight about four times the length of one spiracle a crown of irregularly-spaced rounded tubercles usually extending no more than a quarter of the way down the upper surface (Fig. 6C).Entire surface finely beaded, some of these minute projec tions bear a single apical seta (Fig. 6D)

Fig. 3 .
Fig. 3. Larva of C. tamaulipanum: A -Ventral view of prothorax, mesothorax and metathorax; B -Antenno-maxillary organs and mandibular lobes; C -Abdominal prolegs.D -Anal segment of larva showing two pairs of lappets; E -Spiracular plate of the posterior respiratory process; F -Dorsal view of the posterior respiratory process.

Fig. 4 .
Fig. 4. Puparium of C. tamaulipanum.A -Puparium, dorsal view; B -Operculum with pupal respiratory processes; C -Thoracic respiratory process; D -Spiracular openings of thoracic respiratory process; E -Ornamentation of base of thoracic respiratory proc ess; F -Base of thoracic respiratory process.

Table 1 .
Comparison of the main morphological characters of third-instar larvae of Copestylum species breeding on decaying cactus.

Table 2 .
Comparison ofthe main morphological characters ofpuparia of Copestylum species breeding on decaying cactus.
irregularly-spaced rounded tubercles usually extending no more than a third of the way down the upper, or posterior, sur face.Entire surface finely beaded, each minute projection bears a single apical seta C. isabellina (as V.isabellina) straight length of one spiracle anterior face of horns smoothly -polished, apical portion crowned with conical, spike-like tubercles radiating out in all directions, decreasing in size to numerous rounded tubercles as they descend from the posterior or upper surface C. mexicanum slightly turned twice the length of a front surface bare.The upper and lateral surface with nunerous, (as V.nigra) upward spiracle large, round tubercles.Basal part of the thoracic respiratory processes with a granular surface ++ Characters not described