The complete larval development of the spider crab Maguimithrax spinosissimus (Lamarck, 1818) is re-described and illustrated in detail from laboratory-reared material. The development consisted of the typical pattern reported for the Majoidea, two zoeal stages and one megalopa. The complete larval development from hatching to first crab lasted 5–6 days at temperatures that ranged between 24–28 °C. Both zoeal stages of M. spinosissimus exhibited moderate reduction in the number of setae in the maxilla and maxillipeds, from the first to the second zoeal stage, when compared with other closely related species. Maguimithrax spinosissimus can be easily distinguished from other species belonging to the closely related genus Mithrax by the (i) setation of the endopod of the maxillule, maxilla and second maxilliped in both zoeal stages; (ii) setation of the scaphognathite of the maxilla in the first zoeal stage; (iii) setation of the basis of maxilliped I in the second zoeal stage and megalopa; (iv) morphology of the antennule and antenna in the second zoeal stage; and (v) setation of the antennule, coxal endite of maxilla, and exopod of second maxilliped in the megalopa. All these characters support the recent generic status of Maguimithrax within the Mithracidae. Additional morphological details, not available previously, are provided. This study will provide support for conservation strategies in this species.

The first zoeal stages of the snapping shrimps Alpheus carlae and A. intrinsecus from Brazil were obtained from laboratory-hatched specimens and a detailed morphological description is provided. The morphological characteristics of both species are compared with available descriptions of other Alpheus species with a known first zoeal stage. Remarks from a semi-quantitative comparison of species belonging to A. armillatus species complex (A. carlae and A. brasileiro) and a comparison between larvae of A. intrinsecus from two localities in Brazil are presented. It was possible to distinguish first zoea of both species from the other congeners based on morphology. No intraspecific variability was detected between A. intrinsecus larvae obtained from two locations. Some structures showed differences between species of the A. armillatus complex. However, generalizations and conclusions based on larval morphology for taxonomic rearrangements at this point need to be made with caution and accuracy, since data of a sufficient number of species are not yet available for comparison.

The complete larval development of the lobster shrimp Leonardsaxius amurensis (Kobjakova, 1937) (Decapoda: Axiidea: Axiidae) is described and illustrated for the first time. The first zoeae of this species were collected from the plankton samples and reared in the laboratory before moulting to the megalopa. A molecular genetic analysis based on comparison of partial mitochondrial COI, 12S rDNA and 16S rDNA sequence data confirmed the identity of axiid larvae found in the plankton and L. amurensis adults collected in the same area. The larval development of L. amurensis includes five zoeal stages and a single megalopa. Zoeae I of L. amurensis are characterized by the presence of one short posterodorsal spine on the fifth pleonite in contrast to the larvae of related sympatric species Boasaxius princeps having four posterodorsal spines on the pleonites 2–5. Leonardsaxius amurensis occupies an intermediate position between lobster shrimps with abbreviated pelagic development (2–3 zoeal stages) and species with long development (up to eight zoeal stages). Thus, the number of zoeal stages in the family Axiidae varies widely, similarly to that in the families Callianassidae and Upogebiidae.

Although Parthenopidae is a brachyuran decapod family comprising almost 140 species, there is little knowledge about its larval morphology. There are only two complete larval developments reared in the laboratory and some larval stages described for seven species. In the present work these data are compared and analysed. A summary is made of the larval features that characterize parthenopids that can be used to distinguish them from other brachyuran larvae. In addition, the megalopa stage of Derilambrus angulifrons and Parthenopoides massena was collected from plankton and identified by DNA barcodes. The morphology of the megalopa of D. angulifrons is described for the first time, and that of P. massena is compared with a previous description.

The morphology of the first zoea of Gnathophyllum elegans is described from laboratory-reared material for the first time and analysed in detail with light and scanning electron microscopy. For systematic reasons, morphological characteristics in G. elegans are compared with those in Gnathophyllum americanum, Periclimenes amethysteus, a representative of the subfamily Pontoniinae and the closely related Hymenocera picta (Decapoda: Caridea: Hymenoceridae). We observed differences in the morphology of both Gnathophyllum-larvae, such as the number and arrangement of certain setae. Thus, larvae of the two Gnathophyllum species can be readily distinguished from each other. Further differential diagnosis with P. amethysteus confirms a high similarity between Gnathophyllum-larvae and larvae in the subfamily Pontoniinae, as already mentioned in earlier publications. The systematic relationships of the Gnathophyllidae, Hymenoceridae and Pontoniinae are discussed based on zoeal characters.

Haliclona indistincta has in some respects, a typical reproductive cycle for a marine haplosclerid sponge. We suggest that oocytes originate from archaeocytes and that spermatozoa originate from choanocytes. Oocytes were first seen in November and matured as eggs by May and June. Immature spermatic cysts were identified from February and mature cysts were present in May and June only. Of the individuals surveyed that had reproductive elements present (150/200), reproductive elements from a single sex were reported in over half of the specimens (59%) but there were also many hermaphrodites (41%). Embryos were first seen in June. Larvae were distributed throughout the mesohyl and were released from the end of June to the end of July. Three mobile larval stages and fusing of sibling larvae were observed. Post-settlement stages from early settlement to the development of oscula and excurrent canals are also shown. However, some elements of the larvae (uniform ciliation and no spicules at posterior pole) are not consistent with larvae from this genus.

The muscid Synthesiomyia nudiseta (van der Wulp, 1883) is a species with forensic importance in tropical and subtropical regions of the world. This fly has recently been introduced into southern Europe and, until now, had not been recorded in forensic cases in this region. Here, morphology of all larval instars of S. nudiseta is documented in detail by using a combination of light and scanning electron microscopy. Literature data concerning larval morphology are revised and characters allowing identification from other forensically important Muscidae are listed. The life cycle of this species was studied at four constant temperatures: 15, 20, 25 and 30 °C. Total development varied between 46.50 ± 0.97 days at 15 °C and 15.39 ± 0.32 days at 30 °C. Moreover, we report this species breeding in human corpses, for the first time in Europe, in forensic cases from autopsies at the Anatomical Forensic Institute of Madrid and the Institute of Legal Medicine of Alicante, Spain.

Larvae of eight sea spiders from three families (Ammotheidae, Pycnogonidae and Phoxichilidiidae) are described for the first time. The external morphology of the first larval stage of each species is presented in detail using scanning electron microscopy photographs in order to determine the mode of postembryonic development. Three types of larval development are apparent in the species examined. The species Achelia gracilipes, Eurycyde spinosa, Pycnogonum rickettsi and Pycnogonum stearnsi (families Ammotheidae and Pycnogonidae) have larval morphologies indicative of an ‘ectoparasitic’ development. Morphological characteristics of Achelia simplissima and Achelia chelata (family Ammotheidae) larvae suggest an ‘endoparasitic’ development, while larvae of the species Anoplodactylus viridintestinalis (family Phoxichilidiidae) have traits implying an ‘encysting’ postembryonic mode of development. Larvae of the species Nymphopsis spinosissima have unusual morphological characteristics that may be indicative of a new developmental mode.

Stenopodidae comprises about 28 species in tropical and subtropical waters around the world, but only two of them in the South Atlantic Ocean (SAO)—Stenopus scutellatus and S. hispidus. Larvae of S. scutellatus were never registered in SAO and those of S. hispidus remain poorly researched. The present study fully describes first to seventh zoea of S. hispidus collected in the plankton of SAO, adjacent to Brazil. In addition, eight larvae of S. scutellatus and two larvae of non-identified Stenopodidea from the same area were also described.

The sparetus-group of Entedon is revised. Six species are recognized for the group, including one new species, E. lucasi Gumovsky sp. n., which is described from Algeria. All species are either diagnosed or described, and illustrated, and new distribution records are provided. An illustrated dichotomous key to all species of the group is given. Biology of two species, E. thomsonianus Erdös and E. sparetus Walker, is studied in detail: both species proved to be egg-larval parasitoids of stem-boring weevils, Lixus cardui Olivier on the scotch thistle Onopordum acanthium and Gymnetron asellus (Gravenhorst) on mullein Verbascum sp., respectively. The morphology of larval stages and peculiarities of parasitoid–host relationships are described. Possible benefits of egg-larval parasitism for Entedon species are discussed.

The fly, Philornis downsi Dodge & Aitken, was first collected in 1964 on the Galápagos Islands and is now widespread across the archipelago. Virtually nothing is known about the behaviour and ecology of the fly as well as for the genus in general. Here, we describe all larval instars for the first time, and discuss infection intensity and impacts of parasitism on nestling survival of Darwin's finches. Adult P. downsi are non-parasitic free-living flies, whereas the larvae are obligate blood-feeding parasites on nestling birds. The larvae show a marked shift in their host site specificity – a novel finding for the genus Philornis: the first and early second larval instars live as agents of myiasis in finch nostrils and other tissues, while the older second and third instar larvae reside in the nest material and feed externally on the blood of nestlings, leading to blood losses in nestlings of 18–55%. Pupation occurs in the bottom layer of the nest. The combined effects of tissue damage by the endoparasitic instar larvae and anaemia by nest-dwelling haematophagous instar larvae account for the high nestling mortality (76%) due to Philornis parasitism. This represents the highest mortality by Philornis reported in the literature and emphasizes the extremely serious threat this parasite poses for the endemic passerine fauna of the Galápagos Islands.

The larval morphology of 20 water mite species from Alpine springs is described, with that of Protzia distincta, Tartarothyas romanica, Sperchon mutilus, S. resupinus, S. violaceus, Bandakia concreta, Lebertia cuneifera, L. lativentris, L. sefvei, Atractides adnatus, A. panniculatus, A. macrolaminatus and A. walteri being presented for the first time. For the other 7 species (Panisus michaeli, Panisopsis curvifrons, Thyas palustris, Partnunia steinmanni, Sperchon thienemanni, Lebertia zschokkei and Hygrobates norvegicus), old descriptions exist but, in most cases are not accurate enough to be usable in taxonomy. These species have therefore been re-described. Some general observations on larval morphology and taxonomy are discussed. A short survey of the larval descriptions of West-Palaearctic water mites in general and of spring-living taxa in particular is given, together with a perspective for future research into larval morphology in this geographical area.

The larval stages of Anguillicola crassus, a parasite of the swimbladder of eels, are described, using light and scanning electron microscopy. Second-stage larvae in copepods are characterized by a globular cephalic end and third- and fourth-stage larvae by an apical selerotized ornamentation consisting of two U-shaped median pieces. Third- and fourth-stage larvae are difficult to distinguish from each other using light microscopy because the third moult has never been observed and there are no obvious characters differentiating the two stages. Three characters, however, observed by using high magnification of light and scanning electron microscopy are considered as distinctive between the two stages, namely presence or absence of deirids and lateral alae and differences in the structure of the cuticle. These morphological characters will permit epidemiological studies at the larval level.