Chemical communication involves the production, transmission and perception of odors. Most adult insects rely on chemical signals and cues to locate food resources, oviposition sites or reproductive partners and consequently, numerous odors provide a vital source of information. Insects detect these odors with receptors mosty located on the antennae, and the diverse shapes and sizes of these antennal sensilla are both astonishing and puzzling: what selective pressures are responsible for these different solutions to the same problem- to perceive signals and cues? It gives a brief idea about the selection pressure derived from chemical communication that are responsible for shaping the diversity of insect antennal morphology. In particular, some sorts of new technologies and techniques that offer exciting opportunities for addressing this surprisingly neglected but yet crucial component of chemical communication, are highlighted.

1. ICAR- Indian Agricultural Research Institute
Pusa Campus, New Delhi- 110012
By-
Sourav Chakrabarty
Division of Entomology
ICAR-IARI, New Delhi
Email id- tublu0002@gmail.com
Contact- +91 7001801455
Insect Antennal Morphology: The Evolution of
Diverse Solutions to Odorant Perception

2. INTRODUCTION TO INSECT
ANTENNAE
 First appendicular organs of adult
insects.
 Innervated from Deutocerebral lobes
of insect brain.
 3 major parts of an insect antenna-
 Scape
 Pedicel
 Flagella
 Involved in olfactory perception, hearing, communication, seizing
the preys, gasping female during copulation etc.

3. OBJECTIVES
 To know about mechanisms of Olfactory
perceptions in different insects.
 To know about Odor specific Signal or
Cue detection using different antennal
sensilla.
 To determine costs and benefits of
antennal morphology towards odor
perception, odorant-receptor interactions and
computational modeling.
 To know about evidences in selection of
antennal morphology and its evolutionary
significance.

4. DIVERSITY OF INSECT ANTENNAE
Fig:- From Simple Filiform to Complex Lamellate; from Easy Setaceous to
Modified Bipectinate- Insect Antennae are diverse.

5. Fig:- Antennae of male and female insects with well-developed olfactory sense: (a)
honey bee (Apis mellifera L.); (b) flesh fly (genus Sarcophaga); (c) cariion beetle
(genus Necrophorus); (d) scarabid beetle (genus Rhopaea); (e) saturniid moth (genus
Antheraea); (f) hawk-moth (sphingidae, genus Pergesa) (g) butterfly (genus Vanessa).
INSECT ANTENNAE ARE THE STOREHOUSE
OF OLFACTORY SENSILLA

6. GENERAL CONCEPT OF OLFACTORY
COMMUNICATION
 Most olfactory sensilla are found in the flagellum, with their greatest
densities towards distal ends.
 Antennal morphology is likely to “optimize” rather than “maximize”
odorant-receptor interactions.

7.  More than one receptor could expand the range of activating ligands for the neuron,
allowing it to respond to ligands detected by either receptor (an ‘or’ gate) (Top panel).
 Alternatively, neuronal activation to specific ligands could require activation of both
receptors (an ‘and’ gate), only firing in the presence of both ligands (Bottom panel). In
principle, an ‘and’ gate would impart high selectivity, whereas an ‘or’ gate would
expand the odor response space of the neuron.
OLFACTORY PERCEPTION BY
Drosophila – A Case Study

8. SIGNALS, CUES & ODOR DETECTION
 An Odor is a “Signal” if it influences the behavior of the other
organisms and which is evolved or co-evolved specifically because of
that effect.
 An Odor can be considered as a “Cue” if an incidental source of
information that may influence the behavior of the receiver, despite not
having evolved under selection for that end.

9. UNDERSTANDING SIGNALS vs CUES
Sex Pheromones of moths are
evolved to attract members of the
opposite sex. It is a “SIGNAL”!!
Release of CO2 by human beings
don’t evolve as signal to feeding
mosquitoes, but rather mosquitoes
use CO2 as a “CUE” for locating
victims !!

10.  Both sexes release 25
types of different volatile
organic compounds.
 These aid in mating,
defense and aggregation.
 The organic compounds
are received by the
terminal flagellar segment,
containing 3 types of
olfactory sensilla viz. type
C, type D and type E.
SIGNAL PERCEPTION BY TEMPERATE
BED BUG- A Case Study

11. RHINO STOMACH BOT FLY USES ITS COMPLEX
ANTENNAL SENSILLA TO LOCATE HOST
 Rhino Stomach Bot Fly (Gyrostigma
rhinocerontis) has significantly larger antennae with
more sensilla and sensory pits than any other
Oestridae species, which could be an adaptation to
locate their rare and endangered hosts.

12. COSTS AND BENEFITS OF ANTENNAL
MORPHOLOGY TOWARDS ODOR PERCEPTION
COST
BENEFIT

13. OLFACTORY SENSILLA INCREASES WITH
THE INCREASE IN FLAGELLA LENGTH
 Odorant-receptor interactions are
enhanced primarily by larger
antennae as it can support larger
number of sensilla.
 Usually males have longer
antennae and more sensilla than
those of females.
 Pectinate (Lepidoptera) or
Lamellate (Coleoptera) type of
antennae can support more sensilla
as the surface area is increased. The number of sensilla qualitatively increase
with flagella length across insects

14. ANTENNAL COMPLEXITY &
VOLATILITY OF PHEROMONES
 Beetles with more elaborate antennae use pheromones of low volatility.
 Beetles with relatively simpler antennae use more volatile pheromones.
Fig:- Top 3 species (with simple antennae & pheromones more volatile) and
Bottom 3 species (with elaborate antennae & less volatile pheromones)

15. IDENTITY DETERMINATION THROUGH
ANTENNAL SENSILLA- A Case Study
 Workers of Oecophylla smaragdina, brush their antennae across non-
nestmate workers aggressively .
 The level of aggression towards non-nestmates is positively correlated
with antennal sensilla of workers.
 Large number of sensilla are required for the workers to identify correctly
whether conspecifics are from same or different nests.

16. VARIATIONS IN ODOR PERCEPTION WITH
RESPECT TO ANTENNAL LENGTH IN COCKROACH
 Directionality in odor perception is
lost when maximum removal of antennal
segments are done.
 This arises though loss of total
antennal length across two antennae
rather than antennal symmetry.

17. EFFECT OF ANTENNAL ABLATION IN MATING
OF DIAMOND BACK MOTH- A Case Study
 25% ablation= Not impaired.
 50% ablation= Imapaired.
 If one antenna removed= Directionality is lost

18. ODORANT-RECEPTOR INTERACTIONS &
COMPUTATIONAL MODELLING
Fig:-Predicted concentration (red is high) around the surface of the antennae of nano-
particles (pheromone molecules) in the (a) parallel and (b) ringed arrangement of antennal
scales, and of micro-particles in the (c) parallel and (d) ringed arrangement of antennal
scales. Sensilla number increases with the angle of the scales across genera (indicated by
different colors) of heliozelid moths

19. EVIDENCE OF SELECTION ON
ANTENNAL MORPHOLOGY
 Mate Location & Sexual Selection.
 Anticipatory Investment in Antennal
Morphology
 Social Environment
 Searching for Food
 Searching for Hosts
 Background Noise & Abiotic Modifiers

20. MATE LOCATION AND SEXUAL
SELECTION
 Male insects generally have elaborate, feathery antennae whereas their
conspecific females have simple, filiform antennae.
 Males with more “highly developed” organs of sense would be better
equipped to find the signaling female and thus be at competitive
advantage over other males.
 Females also want “high quality” males who can encounter their
extremely low concentrations of pheromones.

21. ANTICIPATORY INVESTMENT IN
ANTENNAL MORPHOLOGY
 Insect antennae are invested with several qualities according to
their surrounding environment.
 Solitarious locusts of Locusta migratoria have more olfactory
sensilla on their antennae than gregarious adults.
 Gregarious phenotypes can get the senses by the touch with other
locusts.
SOLITARIOUS GREGARIOUS
More olf. sensilla Less olf. sensilla

22. SOCIAL ENVIRONMENT
 Major workers of Oecophylla
smaragdina having more no. of antennal
sensilla (right beside).
 Foraging workers of Tetragonula
carbonaria have well developed olfactory
sensilla (exact below).
 Guards of Tetragonisca angustula have
larger surfac area in their antennae (below).

23. SERACHING FOR FOOD
 According to Chapman, insects with a generalist diet require a
greater number of sensilla than species with a more specialized diet.
 The chemoreceptor sensilla in polyphagus species will definitely
be greater than mono or oligophagus species.
(A & C):- Antennal Olfactory Sensilla of
Dendroctonus rhizophagus
(B & D):- Antennal Olfactory Sensilla of
Dendroctonus valens
 The average number of long basiconic
sensilla is qualitatively greater in strongly
polyphagus D. valens than in oligophagus
D. rhizophagus

24. SERACHING FOR HOST/ PREY
 According to Chapman, Host generalized parasitoids or predators are
predicted to have greater numbers of sensilla than specialist ones.
 However, exceptions are there. Like- Sensilla density in host specialist
Microplitis croceipes is greater than generalist Cotesia marginiventris.

25. BACKGROUND NOISE & ABIOTIC
MODIFIERS
 Not much information has been found whether the intensity of
background olfactory noise (produced by plants or other insects) acts as a
selection pressure on antennal morphology.
 Abiotic factors like- Humidity, Temperature and Air Pollution can
reduce the life span of the odor. Like- Fewer males of Ostrinia nubilalis
take flight in response to female sex pheromones in trials with higher
levels of humidity.
 However, humidity doesn’t affect the response of Ips grandicollis to
synthetic aggregation pheromone.

26. CONCLUSION
 The antennal morphology definitely influences Odorant-receptor
interactions.
 We know a great deal about chemistry and transmission of odors,
and the neurobiology of odorant perception but we know relatively
little about how insects optimize their perception of odors and cues.
 There is a rich seam of potential research questions that address
how size and structure of insect antennae have been shaped by
natural and sexual selection.
 In particular, fluid dynamic modeling coupled with phylogenetic
comparative analyses offer exciting opportunities to understand
precisely how micro-morphological features of antennae can
influence odorant-receptor interactions.

27. REFERENCES
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Ostrinia nubilalis: diel response to sex pheromone and phenylacetaldehyde in
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