Separating Cyana signa, Cyana adita and Cyana dryope based on external morphology (based on Singh et al 2020). 
Males
Cyana dryope: Cyana dryope is similar externally to C. adita, but differs clearly by its broader forewing (apex distinctly acute in adita), larger black discal spots, antemedial line being strongly angled at costa and perpendicular to the anal wing margin (that is oblique in C. adita), and strongly curved postmedial line. 
Cyana adita: Cyana adita is very similar to C. signa, and can be distinguished by its separated distal discal spots (outer discal spots form a “dash” in signa) and slightly more curved and wavy antemedial line (that is oblique and almost straight in C. signa). 
Cyana signa: Cyana signa differs externally from C. adita by its distal discal spots merged into a dash, and oblique antemedial line. 

Females
Cyana dryope female is undescribed. Cyana adita and Cyana signa females are difficult to separate based on external features alone, but Cyana signa antemedial line is mostly straight and oblique while it is more curved and slightly sinuous in Cyana adita.  

List of Cyana Walker, 1854 known from India.
Cyana abrupta Volynkin & Černý, 2019, India (Himachal Pradesh, Sikkim, Arunachal Pradesh, Assam, Meghalaya).
Cyana adita (Moore, 1859), India (North and NE India (Himachal Pradesh, Uttarakhand, North of West Bengal, Sikkim, Assam, Meghalaya, Arunachal Pradesh).
Cyana amabilis (Moore, 1877), India (Andaman & Nicobar Islands).
Cyana arama arama (Moore, 1859), India (NE India (Assam, Meghalaya, Nagaland, Mizoram, Manipur), Sikkim, North of West Bengal, Uttarakhand).
Cyana arama metis Volynkin & N. Singh, 2020, India (North India (Uttarakhand, Punjab)).
Cyana arorai Volynkin, N. Singh, Kirti & Datta, 2020, India (Andaman Islands).
Cyana atlanteia N. Singh, Volynkin, Kirti & Datta, 2020, India (South and NE India (Tamil Nadu, Assam)).
Cyana bellissima (Moore, 1878), India (Uttarakhand, Sikkim, Meghalaya, Arunachal Pradesh, Nagaland, West Bengal).
Cyana bhatejai N. Singh & Kirti, 2015, India (South India (Karnataka, Tamil Nadu), Kerala).
Cyana bianca (Walker, 1856), India (Meghalaya).
Cyana britomartis N. Singh & Volynkin, 2020, India (NE India (Arunachal Pradesh), North of West Bengal, Sikkim).
Cyana candida (Felder, 1874), India (North and NE India (Jammu & Kashmir, Himachal Pradesh, Uttarakhand, Sikkim)).
Cyana catorhoda Hampson, 1897, India (North and NE India (Himachal Pradesh, Uttarakhand, Sikkim, West Bengal, Assam, Meghalaya, Manipur).
Cyana chrysopeleia N. Singh, Volynkin, Kirti & Datta, 2020, India (North and NE India (Uttarakhand, Assam, Arunachal Pradesh, Mizoram)).
Cyana coccinea coccinea (Moore, 1878), India (NE India (Sikkim and north of West Bengal), Assam, Uttarakhand).
Cyana coccinea flavotincta (Draudt, 1914), India (NE India (Assam)).
Cyana conclusa nicobara N. Singh, Volynkin, Kirti & Datta, 2020, India (Andaman & Nicobar).
Cyana detrita Walker, 1854, India (North and NE India (Jammu & Kashmir, Himachal Pradesh, Punjab, Uttarakhand, Sikkim, West Bengal, Meghalaya, Nagaland), Arunachal Pradesh).
Cyana divakara (Moore, 1865), India (NE India (Assam), Sikkim, North of West Bengal, Arunachal Pradesh).
Cyana dohertyi dohertyi (Elwes, 1890), India (Eastern Himalaya (Sikkim, North of West Bengal, Assam, Meghalaya, Nagaland, Arunachal Pradesh, Manipur)).
Cyana dohertyi eirene Volynkin & N. Singh, 2020, India (North India (Himachal Pradesh, Uttarakhand)).
Cyana dryope Volynkin & N. Singh, 2020, India (West Himalaya, North India (Himachal Pradesh)).
Cyana dudgeoni Hampson, 1895, India (Sikkim, North of West Bengal, NE India (Assam, Meghalaya, Manipur, Nagaland, Mizoram, Arunachal Pradesh)).
Cyana flavicincta (Hampson, 1903), India (NE India (Assam, Meghalaya), Nagaland, Arunachal Pradesh).
Cyana gazella (Moore, 1872), India (North and NE India (Jammu & Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, north of West Bengal, Meghalaya, Assam)).
Cyana gelida gelida (Walker, 1854), India (North and NE India (Himachal Pradesh, Uttarakhand, Sikkim, north of West Bengal, Assam, Meghalaya, Nagaland, Arunachal Pradesh)).
Cyana guttifera (Walker, 1856), India (South, North and NE India).
Cyana harterti (Elwes, 1890), India (NE India (Assam, Meghalaya, Mizoram)).
Cyana intercomma Černý, 2009, India (NW India (Jammu & Kashmir)).
Cyana khasiana Hampson, 1897, India (Assam, Meghalaya, Sikkim, Arunachal Pradesh, North of West Bengal).
Cyana linatula (Swinhoe, 1891), India (Rajasthan, Meghalaya, Maharashtra, Tamil Nadu, Karnataka, Kerala).
Cyana moelleri (Elwes, 1890), India (NE India (Meghalaya), Sikkim, North of West Bengal).
Cyana neopuer N. Singh, Bhattacharyya & Volynkin, 2019, India (Meghalaya, Arunachal Pradesh, Sikkim).
Cyana obliquilineata (Hampson, 1900), India (Assam, Meghalaya, Arunachal Pradesh, Sikkim, South India (Karnataka)).
Cyana peregrina (Walker, 1854), India (South India and East India, Uttarakhand); Sri Lanka.
Cyana perornata (Walker, 1854), India (NE India).
Cyana puella puella (Drury, 1773), India (Himachal Pradesh, Uttarakhand, Haryana, Sikkim, Meghalaya, Madhya Pradesh, Chhattisgarh, Maharashtra, Tamil Nadu, Kerala, Andaman & Nicobar Islands, Gujarat, Bihar).
Cyana puer (Elwes, 1890), India (NE India (Assam, Manipur, Meghalaya, Nagaland), Arunachal Pradesh, Sikkim, West Bengal).
Cyana quadrinotata (Walker, 1869), India (NE India (West Bengal, Assam, Meghalaya, Nagaland, Mizoram, Sikkim, Arunachal Pradesh).
Cyana rudloffi Volynkin & N. Singh, 2020, India (Andaman Islands).
Cyana selangorica paeninsulana Černý, 2009, India (NE India, Assam, Mizoram).
Cyana signa (Walker, 1854), India (North and NE India (Himachal Pradesh, Uttarakhand, Sikkim, Meghalaya, Manipur, Arunachal Pradesh, Manipur)).
Cyana sikkimensis (Elwes, 1890), India (NE India (Sikkim and north of West Bengal)).
Cyana trilobata Fang, 1992, India (NE India, Sikkim).
Cyana watsoni Hampson, 1897, India (NE India (Assam, Meghalaya)).