Good alpha taxonomy is central to biology. On the basis of a survey of arthropod studies that used multiple disciplines for species delimitation, we evaluated the performance of single disciplines. All included disciplines had a considerable failure rate. Rigor in species delimitation can thus be increased when several disciplines chosen for complementarity are used. We present a flexible procedure and stopping rule for integrative taxonomy that uses the information from different disciplines separately. Disagreement among disciplines over the number and demarcation of species is resolved by elucidating and invoking evolutionary explanations for disagreement. With the identification of further promising study organisms and of new questions for in-depth analysis, evolutionary biology should profit from integrative taxonomy. An important rationale is clarity in researcher bias in the decision-making process. The success of integrative taxonomy will further increase through methodological progress, taxonomic training of evolutionary biologists, and balanced resource allocation.

The paper integrates two independent studies of numeric morphology-based alpha-taxonomy of the cryptic ant species Temnothorax crassispinus (Karavajev, 1926) and T. crasecundus sp. n. conducted by different investigators, using different equipment, considering different character combinations and evaluating different samples. Samples investigated included 603 individual workers from 203 nests – thereof 104 nest samples measured by Seifert and 99 by Csösz. The material originated from Europe, Asia Minor and Caucasia. There was a very strong interspecific overlap in any of the 29 shape characters recorded and subjective expert determination failed in many cases. Primary classification hypotheses were formed by the exploratory data analysis Nest Centroid (NC) clustering and corrected to final species hypotheses by an iterative linear discriminant analysis algorithm. The evaluation of Seifert’s and Csösz’s data sets arrived at fully congruent conclusions. NC-Ward and NC-K-means clustering disagreed from the final species hypothesis in only 1.9 and 1.9% of the samples in Seifert’s data set and by 1.1 and 2.1% in Csösz’s data set which is a strong argument for heterospecificity. The type series of T. crassispinus and T. crasecundus sp. n. were allocated to different clusters with p = 0.9851 and p = 0.9912 respectively. The type series of the junior synonym T. slavonicus (Seifert, 1995) was allocated to the T. crassispinus cluster with p = 0.9927. T. crasecundus sp. n. and T. crassispinus are parapatric species with a long contact zone stretching from the Peloponnisos peninsula across Bulgaria northeast to the southern Ukraine. There is no indication for occurrence of interspecifically mixed nests or intraspecific polymorphism. However, a significant reduction of interspecific morphological distance at sites with syntopic occurrence of both species indicates local hybridization. The results are discussed within the context of the Pragmatic Species Concept of Seifert (2014). The taxonomic description and a differential diagnosis of T. crasecundus sp. n. are given.

Hybridization in isolated populations can lead either to hybrid breakdown and extinction or in some cases to speciation. The basis of hybrid breakdown lies in genetic incompatibilities between diverged genomes. In social Hymenoptera, the consequences of hybridization can differ from those in other animals because of haplodiploidy and sociality. Selection pressures differ between sexes because males are haploid and females are diploid. Furthermore, sociality and group living may allow survival of hybrid genotypes. We show that hybridization in Formica ants has resulted in a stable situation in which the males form two highly divergent gene pools whereas all the females are hybrids. This causes an exceptional situation with large-scale differences between male and female genomes. The genotype differences indicate strong transmission ratio distortion depending on offspring sex, whereby the mother transmits some alleles exclusively to her daughters and other alleles exclusively to her sons. The genetic differences between the sexes and the apparent lack of multilocus hybrid genotypes in males can be explained by recessive incompatibilities which cause the elimination of hybrid males because of their haploid genome. Alternatively, differentiation between sexes could be created by prezygotic segregation into male-forming and female-forming gametes in diploid females. Differentiation between sexes is stable and maintained throughout generations. The present study shows a unique outcome of hybridization and demonstrates that hybridization has the potential of generating evolutionary novelties in animals.

The world-wide fauna of the genus Cardiocondyla is assessed to have 79 currently recognized species and is subdivided into 14 informal species groups plus three monotypical clades. It is argued that the phylogenetic placement of Cardiocondyla next to the genus Ocymyrmex Emery, 1886 as proposed by Blaimer et al. (2018) is doubtful. The main body of the paper is a species-level taxonomic revision of those six species groups and the one monotypical clade which have their centers of origin in the Oriental and Australasian regions. The fauna of this area comprises 26 species eleven of which are described as new: Cardiocondyla schulzi n.sp., C. argentea n.sp., C. semiargentea n.sp., C. argyrotricha n.sp., C. latifrons n.sp., C. micropila n.sp., C. allonivalis n.sp., C. excavata n.sp., C. goroka n.sp., C. subspina n.sp. and C. sulcata n.sp. Recognized as junior synonyms were: Cardiocondyla tsukuyomi Terayama, 1999 and C. breviscapus Seifert, 2003 of C. minutior Forel, 1899 and C. brevispinosa (Donisthorpe, 1948) of C. paradoxa Emery, 1897. Each confirmed species is characterized by verbal descriptions, 18 numerically recorded phenotypic characters, z-stack images in three viewing positions, the key, a differential diagnosis and, if available, a brief life history. Exploratory and hypothesis-driven data analyses of phenotypic characters support the separation of the cryptic species Cardiocondyla minutior Forel, 1899, C. goa Seifert, 2003 and C. tjibodana Karavajev, 1935, two of which have dispersed as tramp species to the New World. Missing development of postzygotic isolation due to very strong prezygotic barriers is indicated in the closely related species Cardiocondyla latifrons n.sp. and C. micropila n.sp. which produce fertile hybrids in laboratory rearings but apparently do not hybridize in a natural context even if nesting in close neighborhood. The strong prezygotic isolation is supposed to be generated by the prevalence of intranidal mating and the difficulty for a mated heterospecific gyne to become accepted in a foreign colony.