On the Problem of Homology of the Clypeus and Labrum in Coccids (Homoptera, Coccina)

Arguments by P. Pesson and S. Singh on the homology of the clypeus and labrum in Coccina are supported. The labrum rudiment at the apex of the anteclypeus in coccids and reduction of lora in Margarodidae s. l. are considered.

From the earliest works (Weber, 1930;Silvestri, 1934) up to modern times (Koteja et Liniowska, 1978), the anteclypeus of coccids has been interpreted as the labrum, although its correct homology was established already by Pesson (1944) and Singh (1971); the rudiment of the true labrum has remained unrecognized until the publication of Singh (1971). The homology of facial structures can be correctly assessed by comparing the cephalic sclerotization in coccids with that in their sister or, more precisely, parental taxon, i.e., aphids (Shcherbakov and Popov, 2002).
All the Rhynchota possess a specific "rhynchotal" piercing-sucking rostrum, in which the maxillae (their laciniae) and mandibles have been transformed into stylets and enveloped by a modified tubular labium; at the same time, the labrum has become smaller and narrower, so that it covers only the basal portion of the dorsal labial cleft distad to the clypeus (Fig. 2).
Due to reduction of the rostrum proper, i.e., the labium (but not the stylets) in Coccina (Fig. 2, 4), the labrum has also been reduced to a tiny ledge on the distal margin of the anteclypeus. This element is very evolutionarily stable and present in most coccids (see numerous illustrations in Koteja and Liniowska, 1978). This small ledge at the tip of the anteclypeus was apparently overlooked by researchers, including H. Weber, so that the anteclypeus was misinterpreted as the labrum. In their extensive work on facial sclerotization of coccids, Koteja and Liniowska (1978) also regarded the anteclypeus as a labrum, which they called the clypeolabral shield. However, the labrum of Rhynchota is never flanked by the lateral parts of the clypeus and is never shaped as a broad blunt plate that could be confused with anteclypeus; on the contrary, it is the basal part of the anteclypeus that is always located between the tips of the lora, as in aphids (Silvestri, 1934 : 443). It should be noted that the works of Pesson (1944) and Singh (1971) escaped the attention of most researchers and were not usually cited or commented on in reviews. An exception was the monograph on the insect head by R. Matsuda (1965), but he did not discuss the homology of the clypeus and labrum in coccids.
As can be seen in the drawings of H. Weber (1930 : 213), the rostrum of the mealybug Pseudococcus adonidum Geoffroy = Ps. longispinus (Targioni Tozzetti) in the working state is positioned at a right angle to the anteclypeus (designated as the labrum: OL by the cited author); in this case the normally developed labrum would also be flexibly articulated with the anteclypeus. The clypeolabral shield of coccids should be more correctly called clypeoloral, since it incorporates the lora while the labrum is reduced.
Unfortunately, the material of Archaeococcida considered by Koteja and Liniowska (1978) was insufficient and incomplete: some families (Ortheziidae, Phenacoleachiidae) were represented only by adults, others (Margarodidae, with a single exception of Gueriniella Fern.), only by the I instar nymphs, and none of the species was represented by both the I instar nymph and the adult. Since the adult clypeolabral shield of Gueriniella is quite different from the nymphal shields, which are all very uniform, the shield structure seems to change during the final molt to the adult. For this reason, it is not clear how the "clypeolabrum" is transformed. At least with respect to Neococcida, it can be stated that no major transformations of the shield occur at the final molt.
Apart from Orteziidae and Phenacoleachiidae, resembling Neococcida in their facial shield ("clypeolabrum") morphology and showing the smallest deviation from the plesiomorphic variant typical of aphids, the Archaeococcida also include the vast group Margarodidae s. l. with a very peculiar narrow facial shield, which lacks sclerotization in the loral area at least in the nymphs, i.e., the entire shield consists essentially of the clypeus. The epistomal suture in these taxa adjoins the boundary with anteclypeus.
The structure of the facial shield in Margarodidae s. l. suggests that this family is a deviated lineage branching off the basal part of the Coccina tree.

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