Species continuity largely depends on the fitness of the offspring. In insects, for instance, the number, shape and place where eggs are laid are key factors for its protection and survival (Skinner, 1985; Desouhant et al., 2000; Fréchette & Coderre, 2000). Chrysopids are active nocturnal predators during their larval stage (Canard, 2001). For this reason, chrysopids lay their eggs in different substrates near feeding resources, so once larvae hatch, they can easily find prey (Monserrat & Díaz-Aranda, 2012; Monserrat, 2016). Eggs can be found in natural or artificial substrates including leaves, branches, trunks or spines of plants, as well as in human wooden or metal constructions (Duelli, 1986; Gepp, 1989; Monserrat, 2016). Recently, Parejo-Pulido & Mora-Rubio (2019) reported for the first time the use of abandoned spider webs as a new substrate for the oviposition of chrysopid species. They found close-by egg groups on six abandoned webs, one on a web of Argiope lobata (Pallas, 1775) and five on webs of Cyrtophora citricola (Forsskål, 1775). Here, the use of a non-abandoned spider web as an oviposition substrate for Chrysopidae is reported for the first time.

On July 1^st of 2020, a stalked-cluster of 16 eggs was found glued to the edge of an active (i.e. non-abandoned) web of Steatoda nobilis (Thorell, 1875) (Fig. 1). The spider was identified by the author and further confirmation of the species was endorsed by an expert (Dr. Guillem Xavier Pons). The web was located at approximately 2 meters of height, on a pile of stacked rocks that formed an ornamental waterfall in a private pool in the outskirts of Inca, Mallorca (Balearic Islands, Spain) (MGRS coordinate: 31SDD9393, alt. 92 m). Up to six occupied S. nobilis webs were found in the rocky waterfall, but only one of them had Chrysopidae eggs. No trapped adults were found in any web.

These findings raise several intriguing questions, for instance, whether such ovipositioning behaviour gives an adaptive advantage and may increase the fitness of the chrysopid's offspring, as it has been shown in other insect groups (e.g. in moths; Nafus & Schreiner, 1991). If so, this behaviour takes place at the expense of a huge risk to the chrysopid female of entanglement or predation. However, it is still unclear whether chrysopids can discriminate occupied or abandoned spider webs. Moreover, whether future larvae are able to freely move on the web without getting glued or whether they could be able to prey on the spider eggs (as it has been described in other insects, e.g. Mantispidae; Redborg, 1998) should be evaluated. Given that Chrysopidae species differ in their aggregation of egg-clutches (Gepp, 1989), and that the eggs reported by Parejo-Pulido & Mora-Rubio (2019) were laid separately, while the ones found in this note were laid in a cluster (Fig. 1A), it seems that there are at least two different species with such ovipositioning behaviour. Therefore, these findings suggest that oviposition of Chrysopidae on spider webs may have been overlooked.

This previously unreported behaviour calls for future studies, in order to clarify whether: 1) this behaviour happens on all chrysopid species, 2) the spider webs chosen are specific to any chrysopid species, 3) female Chrysopidae are able to distinguish between occupied and abandoned spider webs, 4) webs as ovi-position substrates are effective against egg-predators, 5) this behaviour depends upon the female's past experiences with spiders or webs, 6) this behaviour is linked to local egg-predator density, 7) this behaviour is a result of competition for oviposition substrate. The answers to these questions will contribute to increase the fundamental knowledge on Chrysopidae on unknown aspects of their ecology. Systematic samplings or laboratory studies could be a starting point to better understand these interactions. Nevertheless, even though the relationship between the Chrysopidae oviposition and occupied spider webs should be bore in mind, we cannot rule out the possibility of this interaction been a mere random event in nature.