Split-footed lacewings declined over time: indications from the morphological diversity of their antlion-like larvae

Nymphidae, the group of split-footed lacewings, is a rather species-poor group. Split-footed lacewings nowadays are restricted to Australasia, while fossil forms are also known from other areas of the world, indicating that the group was more species-rich and therefore likely diverse in the past. Split-footed lacewings have rather distinct larvae, roughly resembling antlion larvae, but differing from the latter especially with regard to the mandibles. Antlion larvae usually have three prominent teeth on each mandible, while at least extant larvae of split-footed lacewings only have a single prominent tooth per mandible. Fossils interpreted as larvae of split-footed lacewings are well known from amber from Myanmar (ca. 100 myr; Burmese amber) and by a single specimen from Baltic amber (about 40 myr). We here report additional fossil specimens from Myanmar amber, expanding the known record of fossil forms from six depicted specimens to 15. For the extant fauna, we could compile 25 larvae. We compare the diversity of shape of extant and fossil larvae through time using an outline analysis (based on elliptic Fourier transformation) of the head. The results of this analysis indicate that the morphological diversity, or disparity, of split-footed lacewing larvae was higher in the past than it is today. With this type of analysis, we can show a loss of diversity over time, without the necessity to identify the fossil larvae down to a narrow taxonomical range. A similar pattern has already been recognised in silky lacewings, Psychopsidae. This might indicate a general loss of diversity of lacewing larvae.