1. Introduction
Fossils provide us with a view into the history of life. Fossil remains of animals enclosed in amber have a peculiarly high potential for the investigation of many different aspects of such organisms, including the morphology, development and distribution of today extinct organisms. In contrast to many other types of fossil preservation, inclusions in amber are often exceptionally detailed and preserved in nearly life-like conditions (Arbogast 2007) and with no compressions of the body. A similar detailed preservation, but compressed, can be found in many famous Lagerstätten such as the Burgess Shale (e.g., Gaines et al. 2008; Gould 1990; Haug et al. 2012a).
Especially for the reconstruction of behavioural aspects, amber fossils can provide important insights. Cases of the so-called ‘frozen behaviour’ have been described for fossils of several ingroups of Insecta, such as representatives of e.g. Diptera, Hymenoptera, Coleoptera, Dictyoptera, and Lepidoptera (e.g., Weitschat & Wichard 2002; Weitschat 2009; Boucot & Poinar Jr. 2010; Poinar Jr. 2010; Gröhn 2015; Fischer & Hörnig 2019). These animals were enclosed in the still viscid resin during various actions, such as feeding, mating, egg laying (but note, that most of these cases could reflect false oviposition due to death stress) or capturing prey (Arbogast 2007; Boucot & Poinar Jr. 2010; Poinar Jr. 2010; Gröhn 2015). The resulting fossils represent a kind of snapshots of a specific moment of the life of the animal. Yet, the interpretations of these fossils are not straightforward in every case and so conclusions about behaviour and lifestyle should always be careful discussed (e.g., Grimaldi 1996; Boucot & Poinar Jr. 2010).
Cases of fossils showing euarthropodan species (the larger arthropodan group including insects and their more distant relatives such as other crustaceans and representatives of Euchelicerata) captured during the hatching process are exceptional rare. In Boucot & Poinar Jr. (2010), a piece of Mexican amber with hatching uropodid mites was reported. Gröhn (2015) depicted a group of cockroaches hatching from an ootheca in Baltic amber. Pérez-de la Fuente et al. (2019) described pieces of Lebanese amber containing groups of hatchlings of neuropteran insects (Chrysopoidea) and corresponding remains of eggs. A further example, remarkably not preserved in amber, has been described by Roy & Fåhraeus (1989) from the Early Ordovician Orsten conservation type, corresponding to a nauplius larva, which was fossilised during hatching.
Strikingly, fossil eggs of Insecta have rarely been reported in the literature. Some known examples of such eggs are oothecae, or egg packages of dictyopterans, especially roaches and mantodeans (Boucot & Poinar 2010; Gröhn 2015). One reason for the little knowledge about eggs of extinct euarthropodan species could be that isolated eggs can be difficult to identify in the fossil record. In most cases, clearly identifiable eggs of representatives of Insecta are preserved together with the adult. Examples are fossils of numerous groups such as Diptera, but also Collembola, Lepidoptera and Dictyoptera (e.g., Grimaldi & Engel 2005; Boucot & Poinar 2010; Hörnig et al. 2018; Gao et al. 2019).
Yet, some ingroups of Insecta have a conspicuously characteristic egg morphology. Among these are many ingroups of Lepidoptera or Phasmatodea (e.g., Arbogast et al. 1980; Forister et al. 2006; García-Barros & Martín 1995; Sellick 1997). Another group of extant insects with a remarkable characteristic egg morphology is Reduviidae, the group of assassin bugs, which is an ingroup of Heteroptera, true bugs (Barber 1923; Coscaron et al. 2002; Gil-Santana & Forero 2010; Pikart et al. 2012; Schaefer & Wolf 2003; Shurtz & Mcpherson 2005; Vennison & Ambrose 1990; Voss & Mcpherson 2003). Eggs of Reduviidae are elongated in general shape, often with extensions of the chorion surrounding the prominent operculum forming a collar, are never embedded in plant tissues and frequently arranged in compact cluster in vertical or horizontal position (Southwood 1956).
The group Reduviidae contains about 7,000 formally described species; its representatives occur worldwide, with a major distribution in the tropics (Weirauch et al. 2014). The representatives of this insect group are raptorial and feature a wide range of morphological adaptations to various strategies for capturing prey, such as sub-chelate and chelate raptorial appendages (“legs”) or glands for producing sticking secretions on the legs, as for example in the group Harpactorinae (Weirauch et al. 2014).
Here we report a single piece of Dominican amber containing a group of eggs and nymphs. The latter most likely represent assassin bugs captured in the moment of hatching.