Megastigmus aculeatus (Swederus, 1795)

Nieves-Aldrey, José Luis, Nieves, Maria Hernández & Gómez, José F., 2008, Larval morphology and biology of three European species of Megastigmus (Hymenoptera, Torymidae, Megastigminae) parasitoids of gall wasps, including a comparison with the larvae of two seed-infesting species, Zootaxa 1746, pp. 46-60 : 53-58

Megastigmus aculeatus (Swederus, 1795)

Description: A brief descriptive paragraph of the larva of M. aculeatus , jointly with some biological notes and figures of body and the mandible, was provided by Crosby (1909). A more detailed description is here given. According our observations the larva of this species is similar to the larva of M. pistaciae , except in the following diagnostic features:

Larger size, with a body length of 3.3 mm, and a body width of 1.5 mm. The integument of the body, including the first thoracic segment and the head, are smooth ( Fig. 1 F). The body (lateral view) ( Fig. 2 D) has visible antero-dorsal protuberances between body segments 3–8 and spiracles present from T2–A7. Head (anterior view) ( Fig. 3 F) 1.2 × as wide as high. Antennae appear as irregular, rounded, protruding areas situated closer to the vertex than to the clypeus. Upper margin of vertex entire; a groove and a medial frontal pit absent. Distance between the antero-medial setae of vertex 1.8 × as distance between antennae; antero-medial setae of antennal area situated at the level of the antennae; the clypeal setae and latero-clypeal setae are visible. Labrum rectangular, about 3 times as wide as long, its anterior margin has about 20 small irregular acute lobes, disposed in 2–3 rows ( Figs 3 F, 4F). Maxillae are well differentiated from labium, not divided into parts or lobes; one third of the inner surface of the maxillae is composed of many small irregular protuberances ( Fig. 4 F). Mandibles have four blunt teeth ( Fig. 5 F).

Material examined: ex seeds of Rosa sp. (28 seeds dissected): Spain, Madrid, Valle de S. Pedro, 1/iv/ 2006, JLN-A leg. (n=1)

Remarks. The larva of this species was studied from a single individual extracted from a seed of Rosa in Spain ( Fig. 6 H). There are two native species of Megastigmus associated with Rosa recorded from Europe: M. aculeatus Swederus and M. rosae Bouček, 1971 , but only the former has been recorded from Spain ( Roques & Skrzypczynska 2003). The identity of the species is suggested by rearing of 34 adults from the samples containing infested seeds from which the larva was extracted.

Figures

FIGURE 1. Ventral view of final-instar larvae of Megastigmus. A. M. dorsalis ex Andricus kollari (asexual). B. M. dorsalis ex Andricus grossulariae (sexual). C. M. dumicola ex Plagiotrochus kiefferianus (asexual). D. M. stigmatizans ex Andricus kollari (asexual). E. M. pistaciae ex seeds of Pistacia lentiscus F. M. aculeatus ex seeds of Rosa sp.

FIGURE 2. Lateral view of final-instar larvae of Megastigmus. A. M. dorsalis ex Andricus kollari (asexual). B. M. dumicola ex Plagiotrochus kiefferianus (asexual). C. M. stigmatizans ex Andricus kollari (asexual). D. M. aculeatus ex seeds of Rosa sp.

FIGURE 3. Anterior view of the head of the final-instar larvae of Megastigmus. A. M. dorsalis ex Andricus kollari (asexual). B. M. dorsalis ex Andricus grossulariae (sexual). C. M. dumicola ex Plagiotrochus kiefferianus (asexual). D. M. stigmatizans ex Andricus kollari (asexual). E. M. pistaciae ex seeds of Pistacia lentiscus F. M. aculeatus ex seeds of Rosa sp. Arrows point at antennal setae and anterior margin of clypeus.

FIGURE 4. Anterior view of mouthparts of final-instar larvae of Megastigmus. A. M. dorsalis ex Andricus kollari (asexual). B. M. dorsalis ex Andricus grossulariae (sexual). C. M. dumicola ex Plagiotrochus kiefferianus (asexual). D. M. stigmatizans ex Andricus kollari (asexual). E. M. pistaciae ex seeds of Pistacia lentiscus F. M. aculeatus ex seeds of Rosa sp.

FIGURE 5. Mandibles of the final-instar larvae of Megastigmus. A. M. dorsalis, left mandible anterior view. B. M. dorsalis left mandible, posterior view. C. M. pistaciae, right mandible, posterior view D. M. dumicola, left mandible, anterior view E. M. dumicola, right mandible, anterior view. F. M. aculeatus, right mandible, posterior view G. M. stigmatizans, left mandible, anterior view. H. M. stigmatizans, right mandible, anterior view.

FIGURE 6. A. Cross section of a gall of Andricus kollari showing a larva of M. dorsalis in a secondary cell of the inquiline Synergus umbraculus, and the central cell occupied by the larva of M. stigmatizans. B. Larva of M. dorsalis in a peripheral cell of a gall of A. kollari. C. Larva of M. dorsalis in a gall of Andricus grossulariae (sexual gen.). D. Crosssection of a gall of Plagiotrochus kiefferianus with larvae of M. dumicola E. Larva of M. stigmatizans. F. Detail of head of the larva of M. stigmatizans. G. Cross section of a gall of A. kollari showing the central cell with a larva of M. stigmatizans. H. A seed inside a hip of Rosa sp., attacked by a larva of M. aculeatus.