First record of Mymarommatidae (Hymenoptera) from the Galapagos Islands, Ecuador

1 Introduction

Species of Mymarommatidae (Hymenoptera) are fascinating insects with a body length of less than one millimeter (Gibson et al. 2007; Vilhelmsen and Krogmann 2006). They are distinguished morphologically from other Hymenoptera by a two-segmented petiole, spatulate fore wing with a mesh pattern on the membrane, and with long marginal setae that arise from the inside of the membrane (Gibson 2006). Additionally, the hindwing is reduced to a hooked stub, and the back of the head is formed by a plate joined to the rest of the head by a pleated, bellows-like membrane. The family is almost worldwide in distribution and specimens are found in different ecosystems, especially forests (Huber et al. 2008; Noyes 2019). However, due to their small size and pale color, individuals are rarely noticed, even when they are collected (Gibson 2006). They were presumed to be egg parasitoids (Gibson et al. 2007; Huber et al. 2008; Yoshimoto 1984) and that was finally demonstrated when they were reared from Psocoptera eggs (Honsberger et al. 2022).

Villemant et al. (2023) reported 19 described species in five genera within Mymarommatidae, of which two genera and seven species are only known from fossils. Since Gibson et al. (2007), nine species of Mymarommatidae have been described: Mymaromella pala Huber & Gibson; Mymaromella palella Huber & Gibson; Mymaromella ella Triapitsyn; Mymaromma heptafuniculata Manickavasagam & Ayyamperumal; Mymaromma ignatii Prashanth & Veenakumari; Mymaromma longipterus Manickavasagam & Ayyamperumal; Mymaromma manipurense Manickavasagam & Ayyamperumal; Mymaromma menehune Honsberger & Huber; and Mymaromma shivajiense Manickavasagam & Ayyamperumal. The only species described for the Neotropical region is Mymaromella cyclopterus (Fidalgo & De Santis), recorded in Argentina, Trinidad, Venezuela, and Brazil (Bragança et al. 2004; Fidalgo and De Santis 1982; García 2000; Gibson et al. 2007; Penteado-Dias and Pasenow 2002); however, the genus Mymaromma also has been found in the Neotropics (Rodrigues et al. 2014).

We report the family Mymarommatidae (Mymarommatoidea) for the first time from the Galapagos Islands. This Pacific archipelago is recognized for its unique biological diversity with a high level of endemism (Boag and Grant 1981; Epler 2007; Grenier 2012; Johnson and Raven 1973), nonetheless the parasitoid Hymenoptera have been a poorly studied group (Causton et al. 2006).

2 Materials and methods

In 2018, intensive sampling was carried out following the methodological approach proposed by Longino and Colwell (1997) on the Santa Cruz, Fernandina and Isabela Islands (Alcedo and Darwin volcanoes), followed by sampling in 2019 in Isabela (Puerto Villamil, Sierra Negra volcano), San Cristobal and Floreana. In 2021, the investigation was expanded to Genovesa, Marchena, Pinta, Santiago and Española. For each island, four zones were established, based on Jackson’s (1993) classification, by altitude and vegetation type (coastal, dry, transition, and humid). Three sampling methods were used in each area: malaise traps (three units for 8 days), yellow pan traps (five units for 3 days), and collection with a structured entomological net (four units of 50 double passes). The total sampling effort translated into 146 Malaise units, 208 yellow pan traps units, and 158 entomological net collection units. The specimens collected, were preserved in ethanol and deposited in the Museo de Entomología of the Escuela Superior Politécnica de Chimborazo (ECESPOCH) in Riobamba, Ecuador and in the Terrestrial Invertebrate Collection of the Charles Darwin Research Station (ICCDRS) in Santa Cruz, Galápagos, Ecuador.

3 Results

Three specimens of an unidentified species of Mymaromma (Hymenoptera: Mymarommatidae) (Figure 2A–D) were sorted from the samples collected on Floreana, Isabela and San Cristóbal islands (Figure 1) in Malaise and pan traps from littoral and dry zones between 11 and 25 m a.s.l.

Material examined. ECUADOR – Galapagos Islands: • 1♂; Floreana; 1.27032 °S, 90.48870 °W; 11 m a.s.l.; 29 May 2019; J. Avendaño, D. Albuja leg.; littoral zone; Malaise trap; ECESPOCH. • 1♀; Isabela, Camino al Muro de las Lágrimas; 0.95855 °S, 90.99380 °W; 10 m a.s.l.; 6 June 2019; J. Avendaño, D. Albuja leg.; littoral zone; pan trap; ECESPOCH. • 1♂; San Cristóbal, Puerto Chino; 0.92352 °S, 89.42980 °W; 25 m a.s.l.; 21 June 2019, J. Avendaño, D. Albuja leg.; dry zone; Malaise trap; ICCDRS (Figure 2).

Figure 1:

Location of Mymaromma sp. and distribution of vegetation zones in the Galapagos archipelago following Jackson (1993).

Figure 2:

Specimens of Mymaromma sp. (A, B) Male. (C) Female. (D) Illustration of female Mymaromma sp. Scale bar: 0.5 mm.

4 Discussion

The three specimens of Mymarommatidae found among 2.446 Hymenoptera individuals from 512 samples, using different methods, show how they are rarely collected. Despite our efforts, more specimens may have been present in the samples, but given their small size and pale color they may have been overlooked. Our specimens represent the second series collected from the smaller islands of the Pacific Ocean. Whether they represent the same species described in the Hawaiian Islands (Honsberger et al. 2022) with which they are morphologically similar, or originated from somewhere in mainland South America (likely Ecuador), or elsewhere, is unknown.

Previous records of Mymarommatidae in the Neotropical region include four species from Argentina, Bermuda, Trinidad, Venezuela and Brazil (Fidalgo and De Santis 1982; García 2000; Gibson et al. 2007; Penteado-Dias and Pasenow 2002; Rodrigues et al. 2014). The discovery of Mymarommatidae, particularly the genus Mymaromma, in the Galapagos Islands represents a significant finding that expands our understanding of their distribution in both South America and remote island ecosystems.

Huber et al. (2008) and Honsberger et al. (2022), respectively, predicted and reported parasitism on Psocoptera eggs by Mymarommatidae. In the Galapagos Islands, 44 Psocoptera species from different islands, altitudes, and habitats (littoral and dry) have been documented (Thornton and Woo 1973). Considering the presence of native and endemic Psocoptera species (Charles Darwin Foundation 2024; Thornton and Woo 1973), it is possible that the Mymaromma species we recorded may be endemic, that other species might also be present, or that they may have been introduced but parasitize endemic hosts. The capture of more specimens, especially females upon which the species-level taxonomy is based, will allow us to make the necessary slide mounts for taxonomic identification at the species level.

We showed that the use of several sampling methods in a variety of habitats is effective for collecting these tiny and cryptic hymenopterans (Gibson et al. 2007; Huber et al. 2008; Longino and Colwell 1997). Additional field collections are needed to obtain and identify new parasitoid species, study their interactions with their hosts, and determine how the presence of invasive species or anthropogenic impacts affects them in general. This knowledge will be essential for developing effective conservation strategies in these fragile island ecosystems (Causton et al. 2006).