N. tenuis. These species are characterised by infundibular upper pitchers and highly viscous pitcher fluid. Nepenthes inermis is thought to be most closely related to N. dubia. However, it is easily distinguished from that species as it completely lacks a peristome in its upper pitchers unlike the latter. The pitchers are also usually green, while those of N. dubia tend to be yellow to orange in colour. In 2001, Charles Clarke performed a cladistic analysis of the Nepenthes species of Sumatra and Peninsular Malaysia using 70 morphological characteristics of each taxon. The following is part of the resultant cladogram, showing "Clade 1", which has 51% bootstrap support. Its most strongly supported subclade is the sister pair of N. inermis and N. dubia, having 95% support. In his description of the Bornean species N. campanulata, botanist Shigeo Kurata suggested that it may be closely related to N. inermis. However, other taxonomists disagree with this hypothesis. While these two species are similar in general stature and pitcher morphology, they are not only separated by great geographical distance, but also occur in completely different habitats; N. campanulata is a lowland species endemic to limestone substrates, whereas N. inermis usually grows as an epiphyte at elevations of . Natural hybrids Nepenthes × pyriformis Nepenthes inermis is known to hybridise with N. talangensis on the upper slopes of Mount Talang, where the two species grow sympatrically. N. talangensis was only described as a distinct species in 1994. Prior to this it was placed within N. bongso and some of the older literature identifies this hybrid as N. bongso × N. inermis. Nepenthes inermis × N. talangensis has been the subject of taxonomic confusion in the past. In an article published in 1973 on the Nepenthes of Borneo, Singapore, and Sumatra, Shigeo Kurata incorrectly identified specimens of this hybrid as belonging to N. dubia. In 1997, Matthew Jebb and Martin Cheek published their monograph "A skeletal revision of Nepenthes (Nepenthaceae)", in which they referred to N. dubia plant material from Mount Talang (Kurata s.n. SING). Charles Clarke later identified Kurata s.n. as representing N. inermis × N. talangensis. The natural hybrid is similar to N. dubia, but can be distinguished on the basis of several stable characters. The hybrid has a wider pitcher lid that is never relfexed beyond 90 degrees and the pitcher cup is not appressed in the lower parts as in N. dubia.